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RV Toilet Leaking? Here’s What to Do!

RV Toilet Leaking? Here’s What to Do!

There’s not a lot that’s worse than finding your RV toilet leaking (except maybe finding your black tank leaking!), but it helps to know what’s causing the leak so that it can be repaired as quickly as possible.

In today’s post, we’re going to run through the various reasons why an RV toilet might be leaking, and what you can do to troubleshoot your leak, and repair it.

What Would Cause an RV Toilet to Leak?

The first thing to do when you find a leak that appears to be coming from your toilet is to investigate the area to find out exactly where the leak is coming from and when it’s happening.

For example, is the water dripping from the upper part of the toilet when you flush it? Is the leak happening when the toilet bowl is full? Are you finding water around the base of the toilet?

Once you’ve determined the origin of the leak, you’ll be able to spring into action to repair it.

RV toilets usually leak for one of these reasons:

Bad Flange Seal

This is the seal between the toilet and the floor (like the wax seal under your home toilet). From time to time, these seals can come loose or develop gaps that cause leaking.

Here’s an example of a Dometic flange seal followed by a standard (supposedly universal) 3” flange seal:

Dometic 385311267 Floor Flange Seal Kit
  • Floor flange fits 110 and 210 Series Domestic RV toilets
  • Seals to prevent leaks
Thetford 33239 Closet Flange Seal , Gray
  • Closet flange for Thetford toilets.Fit Type: Universal Fit
  • Replaces 02125

Cracked Water Valve

A cracked water valve could allow water to continue flowing into your toilet bowl, eventually causing it to overflow. To see a water valve being replaced in a Dometic RV toilet, check out this video from RV Living Yet.

This is a Dometic Sealand water valve kit:

Dometic 385311641 Genuine OEM Water Valve Kit | 300 301 310 320 Dometic and Sealand Toilets | Includes Hose Clamp and Fasteners | White Compact Kit for Efficient Repair and Replacement
  • QUALITY REPLACEMENT PARTS: The Dometic 385311641 Sealand Water Valve Kit is designed to fit 300 Series Dometic toilets and includes all necessary...
  • COMPLETE KIT: The package comes with a water valve, hose clamp, fasteners, and instructions to ensure hassle-free setup.

Bad Valve Seal/Flush Ball Seal

The “valve seal” or “flush ball seal” is the seal inside the toilet bowl that holds water (and the other contents) until you flush… AND keeps odors from the black tank below from coming up into the livinst area of your RV.

Dometic 385311658 OEM Flush Ball Seal Kit | for Use with 300/301 / 310 Series Toilets
  • GENUINE OEM: Spare Parts for reliable and long-lasting performance
  • Country of Origin : China

Cracked Bowl

This is more likely to happen with an RV toilet that has a plastic bowl. A small crack may have developed that allows water to drain out of the bowl during use.

Dometic 320 RV Toilet | White | 302320081 | Standard Height | Gravity Toilet | Elongated Ceramic Bowl | Flush with Foot Pedal | For RVs, Trailers, and Outdoor Campers
  • EASY TO CLEAN: Dometic's 320 standard height toilet boasts a deep, 100% vitreous ceramic bowl designed to facilitate effortless cleaning and...
  • AT HOME COMFORT: The Dometic 320 toilet goes beyond mere functionality, offering an elongated residential-style design that brings the comforts of...

Loose or Cracked Fittings

Here you would need new fittings or you may only need to re-wrap the current fitting/fittings with new Teflon tape and tighten the fitting.

What is the Most Common Reason for an RV Toilet to Leak?

The most common reason for an RV toilet to leak is a bad flange seal.

The flange seal is the rubber ring that seals your RV toilet to the floor. Also called a closet flange seal, this seal is similar to the wax seal under your home toilet. Over time, movement from driving and flexing from the weight on the bowl can cause this seal to fail.

Your first step, though, is to confirm that the bolts holding the toilet to the floor are secure. Your “leaking flange seal” may actually be caused by loose bolts. If that doesn’t solve the problem, and your issue is that your toilet leaks at the base when you flush, you’ve likely got a bad flange seal. Let’s take a look at how to replace it, DIY style.

How Do I Replace the Flange Seal on my RV Toilet?

You’ll need the following items:

  • Replacement flange seal – this one fits a standard 3” ring:
Thetford 33239 Closet Flange Seal , Gray
  • Closet flange for Thetford toilets.Fit Type: Universal Fit
  • Replaces 02125
  • Socket set or wrench
  • Towels
  • Putty knife or utility knife to remove the old flange seal
  • Teflon tape
  • Rubber gloves
  • Cleaning supplies

Replace the old flange seal by taking the following steps:

  1. Shut off the water supply to the toilet (and/or RV) and be sure the water pump is turned off
  2. Disconnect the toilet from the water supply (locate the water pipe and unscrew it, freeing the water pipe from the toilet… it’s likely hidden up behind the toilet)
  3. Locate the bolts holding the toilet to the floor and remove them. Pull the toilet straight up off the floor.
  4. Locate the old flange seal. (Note: It may be stuck to the bottom of the toilet.) Remove the old flange seal and cover the sewer hole with an old rag or plastic bag (to prevent odor).
  5. Clean the bottom of the toilet bowl and the floor around the sewer ring very well.
  6. Install the new toilet flange seal.
  7. Remove the bag or rag from the sewer hole and lift the toilet carefully onto the flange seal, being careful to place the toilet in line with the bolts.
  8. Using your wrench, replace and tighten all of the bolts that hold the toilet to the floor.
  9. Reconnect the water, and test for leaks by flushing the toilet a few times.

What’s Wrong With My RV Toilet? Water Doesn’t Stay in the Toilet Bowl?

If water doesn’t stay in the bowl between flushes, then you’ve got a bad valve seal (also referred to as a flush ball seal). This is the black seal that you can see by looking into the toilet. Its purpose is to seal the blade valve that holds water in the bowl and opens to allow the contents of the bowl to flow into the black tank.

These seals can dry out over time and can crack and cause gradual leaking of the water out of the toilet bowl (and/or odors from the black tank back into the bathroom). To prevent this from happening you can occasionally treat the seal with a conditioner/lubricant to keep the valve sea operating smoothly and prevent it from drying out.

Thetford RV Toilet Seal Lube and Conditioner - Toilet Seal Lubricant - 24 oz 36663
  • [VERSATILITY]: Thetford's RV Toilet Seal Lube & Conditioner works with all permanent or portable toilets found in RVs and boats
  • [FUNCTIONALITY]: Penetrates the toilet seal, lubricating and protecting

This is an example of a valve seal/flush ball seal, but this one may not fit your RV toilet. Again, you’ll need to check the make and model of your RV toilet, and then order the appropriate seal to fit.

Dometic 385311658 OEM Flush Ball Seal Kit | for Use with 300/301 / 310 Series Toilets
  • GENUINE OEM: Spare Parts for reliable and long-lasting performance
  • Country of Origin : China

How Do I Change the Valve Seal/Flush Ball Seal of my RV Toilet?

The typical process for replacing the flush ball seal would involve dismantling the toilet to get to the valve. You’ll want to refer to your RV toilet’s owner’s manual for the exact procedure for your make & model. Here’s an example of our friend Brian at replacing the valve seal on his Thetford toilet:

However, some RV toilets, specifically the newer Dometic toilets, allow you to replace the flush ball seal from above without dismantling the toilet, making it a much easier repair.

Why Would an RV Toilet Leak from the Top?

If the toilet is leaking from the top portion during the flush cycle, then you may have a bad float seal in the vacuum breaker. In this case, you may be able to buy a kit to replace the float seal or take the current float seal mechanism apart to be thoroughly cleaned.

Let’s take a look…

How Do I Know If My RV Toilet Vacuum Breaker is Leaking?

Your RV toilet’s vacuum breaker can leak due to age or wear & tear. If water is leaking from the upper part of the toilet, and especially if the back of the toilet is wet, the problem may be the toilet’s vacuum breaker.

Note: the float seal can also leak from the same area, so you need to determine whether the leak is caused by the float seal or the vacuum breaker. To do this, look inside the toilet bowl. If the problem is the float seal, it’s likely to appear cracked, dried out, or damaged in some way. If the float seal looks good, your vacuum breaker is the likely culprit of water leaking from the upper part of the toilet.

How to Repair a Leaking RV Toilet Vacuum Breaker

  1. Locate your RV toilet’s model / serial number near the bottom of the toilet. There are parts that are universal, but having your toilet’s make and model number will allow you to make sure the parts will fit your toilet before ordering.
  2. Shut off the water supply to the toilet (but have a towel on hand for any excess water).
  3. Locate the hose that connects the toilet to the supply line near the foot pedal valve, and loosen the hose clamps.
  4. Slide the hose off and pull out the vacuum breaker.
  5. Identify the rubber grommet holding the vacuum breaker to the toilet and remove the grommet with your finger. (Stick a finger into the rubber grommet and pull it out.)
  6. Clean the area well.
  7. Install the new rubber grommet and insert the new breaker.
  8. Reattach the hoses (new hoses may come with your kit), and turn the water supply back on.
  9. Test for leaks by flushing the toilet.

What If My RV Toilet Leaks When the Bowl is Full of Water

This may be an indication of a cracked toilet bowl, especially if you have a plastic toilet as this is more common with plastic toilets than with porcelain bowls.

Check the connections behind the toilet and make sure the leak is not coming from these connections.

If you determine that the toilet bowl is cracked, you’ll need to replace the entire toilet (and maybe you’d like to consider replacing your standard RV toilet with a composting toilet instead?).

For More Information…

For more information on RV toilets in general, feel free to check out our post and video entitled “RV Toilet Talk: How Does an RV Toilet Work?”, and we’ll help you to flush out the details! (Sorry – we couldn’t help ourselves!)

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Tuesday 21st of November 2023

I have a Thetford mod 31687 toilet and everyone once in awhile water will come out of the top of the water valve. There is a hole in the top of it, I'm assuming it's a pressure relief or overflow valve. Do I need to just replace it or is there something else that might be causing water to come out of the top?


Monday 3rd of July 2023

Why do we hear water spraying after the old owner said he just replaced the toilet with an new one no water on floor but continue hearing water spraying


Monday 3rd of July 2023

That sounds strange, @Scott. Does it happen 100% of the time? Or only when flushing the toilet? Does the toilet have a handheld rinsing wand? If so, be sure to trace the hose and check the connections as one could be loose. May take some digging and sleuthing, but you're definitely going to want to confirm that what you're hearing ISN'T a leak somewhere.

One totally random thought... does your RV have a black water tank flushing system? If so, there's usually a water hose hookup for it in the fresh water compartment. Are you SURE you didn't accidentally hook city water to that attachment point, thinking it was the city water connection? If you did... you'd hear water running all the time as the tank gets rinsed... and, eventually, overflows when it fills up.


Thursday 22nd of June 2023

When I then on my water heater to take a shower water is running out from behind my toilet .


Friday 23rd of June 2023

Oh boy, Pam. Does it happen right away? If so, it sounds like a fitting or pipe may be cracked/broken.

Paul Stevens

Thursday 8th of June 2023

Hello, Im a retired engineer helping a friend with a leaking RV toilet, we think its a Sealand 320 Dometic! But not absolutely sure. Water started to seep from under the toilet. I removed the toilet and with a new seal have secured it. I noticed the flange/baseplate had a split so I would like to replace this but I'm just not sure that once I’ve removed the retaining screws if it is a threaded R/H L/H pull off?? Dometic uk have been so unhelpful

Paul Stevens

Friday 9th of June 2023

@TheRVgeeks, Hey, Thanks guys this is very helpful! I think our toilet is a 300 and it looks like we will have to get underneath the caravan to see how this is all fitted (A messy job I suspect) we do need to fit a new flange as the current one is split! How it got split ill neve know but I suspect when someone tried to lift it previously they split it. Having removed the screws I cannot move it and think you are right in suggesting it could be glued in. As an Engineer (No arrogance intended) but its perhaps appropriately a “crap” design.

Another Q. If I may? Since I re assembled this with silicone in surgical dry conditions and left 24 hours to set and it was leak free for a week. Im just wondering do you know? Could it be that the seal on the release valve is leaking and running down the outside of the waste pipe thus giving us a false impression that the leak is the waste pipe? The leak is minor and doesn’t seem to get progressively worse with more use.

Totally baffles as Ive repaired plenty of domestic toilets previously without issue.

Thanks for your assistance with this, it is appreciated.


Thursday 8th of June 2023

Hi Paul. Good question... we're not 100% sure, as we've never removed the floor flange that attaches an RV toilet to the floor and connects it to the plumbing. Looking online at replacement Dometic toilet floor flanges (we looked here on e-Trailer just to get an idea: it could either be threaded OR glued. Depends on how the RV factory installed/attached it.

Here's a link to the Dometic Installation PDF instructions for the Model 320 toilet (which also includes information on where the model/serial number label is, which might help you confirm that it's a 320), that may help you to sleuth out how the various components are installed:*wiu6a7*_ga*MTIyODYyOTUzMS4xNjg2MjU3NjAz*_ga_8HB6BZ1XMT*MTY4NjI1NzYwMy4xLjEuMTY4NjI1NzYyMi4wLjAuMA..

But, it may come down to having to try removing that floor flange to see how, exactly, the factory attached it (and HOPE that it's threaded, not glued). ????


Thursday 4th of May 2023

In addition to my previous post about my toilet leak - I do have someone coming to look at it but not until Monday. What should I do in the meantime?


Thursday 4th of May 2023

Hi Marion. So sorry to hear about the problem you’re having. Yes indeed, you want to turn off the water pump in your RV to prevent any more water from coming out through the leak. Unfortunately, there’s no way around that, which means you won’t have water available in your rig until Monday when the repairman comes.

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