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Is It OK to Leave the Gray Tank Valve Open On My RV?

Is It OK to Leave the Gray Tank Valve Open On My RV?

If you’ve stayed at campgrounds and RV parks with full hookups, you may have found yourself wondering, “Is it ok to leave the gray tank valve open?”

That’s a great question… with a bit of a controversial answer.

One of the luxuries of staying in a full-hook-up campground is the ability to use water freely for showers, cooking, and washing dishes, as the water runs freely. The sewer connection allows you to easily let water right out of the RV instead of keeping the gray tank closed and having it fill up.

You’d think that leaving your RV’s gray valve open when fully hooked up would be great because you can take long showers without worrying about filling up your gray tank, right? (At least as long as you have enough hot water.)

But many people leave the gray valve closed because they believe that leaving it open will allow sewer smells to come up into their RV from the campground sewer system.

In this post, we’ll show you how to leave your gray tank open without suffering through the experience of sewer odors.

Does Leaving the Gray Tank Valve Open Cause Odors In an RV?

Everyone expects RV black water to stink. But many aren’t aware of how bad the smell of gray water can be. After all, it’s just sink and shower water, right?

Well, when you mix everything that goes down your shower and sink drains (including food particles, toothpaste, body oil, soaps, etc), gray water can quickly become pretty nasty. That’s especially true during long boondocking trips when RVers severely limit water use.

However, since the drains under every sink and shower in an RV are equipped with a p-trap, leaving the gray water valve open shouldn’t allow odors into the RV. The water in the traps blocks odors from entering the RV’s living space.

A p-trap under and RV sink shown

This is the trap in the plumbing system (called a “p-trap”) where water stays, effectively blocking odors from coming up into the RV.

Long periods without use can allow the water in the p-traps to evaporate, and driving on twisty or bumpy roads can cause it to slosh out. But both of these situations are easily remedied by running a small amount of water down each drain.

However, there is a pipe with a direct connection to the sewer hose that leads to the RV holding tank vents on the roof of the RV.

This can allow air from the sewer system to come up through the roof vents.

Two tank vents on our RV roof

There’s a pipe with a direct connection to the sewer hose that leads to these tank vents on the RV’s roof, which can allow air from the sewer system to come up through the roof vents.

If you’ve ever smelled sewer odors in a full hook-up campground, it’s possible that someone’s RV is venting the park’s sewer system out through their roof.

But if you leave your gray tank valve closed, you’ll need to monitor the tank and dump it as needed — every couple of days, or more, depending on how much you focus on conserving water. And that would be the case even if you’re staying at a full hook-up RV park.

A Newmar RV tank monitor panel

If you close your gray tank valve, you’ll need to closely monitor how full your gray tank is, and manually dump it as it approaches full.

It seems a shame to have to do that when you’re hooked up to the RV park sewer system. When we settle into a full hook-up for a while, part of the luxury of being there is to not have to worry so much about water for a change (unlike when we’re boondocking).

We want to do laundry on board, shower without drizzling water, etc. Sometimes it’s just nice to be able to forget about monitoring water usage so closely.

How to Keep Sewer Odors from Coming Up Through the Roof Vents

If you want to keep your gray tank valve open but you want to keep sewer odors from coming up through your roof vents, there’s a very simple solution.

All you need to do is leave a section of your sewer hose hanging off the end of your sewer hose support.

What this does is create a whole-house p-trap in that small section of hose that will hold accumulated water. That prevents odors from coming up out of the RV park’s sewer system and making their way through your sewer hose and up onto the roof (or into the RV interior).

To see this process visually, have a look at our YouTube video demonstrating how we do it:

This trick will enable you to enjoy the full benefits of full hook-up sites without having to constantly monitor and dump the RV gray tank, and with no sewer odors. And as far as “sewer flies” go, we’ve heard a few stories about them, but have never once seen one in 20 years of full-timing with the gray valve open at full-hook-up campsites.

If you don’t have an RV sewer hose support, (or you need a new one), we highly recommend the Valterra Slunky. Ours works great and is still like new after years of use.

Valterra S2000 Slunky RV Drainage Hose Support, 20 ft., Black
  • RV SEWER HOSE SUPPORT: Elevated design supports your drainage hose and promotes better draining
  • IMPROVED DESIGN: 26% wider and 30% deeper for increased stability and a smoother overall RV experience

If you’re interested in our (awesome) 360 Siphon roof vents, here’s a link, as well as a video tutorial on how to install them.

Lippert 360 Siphon RV Roof Vent Cap for Odor Prevention with Universal Fit, Built-In Bug Screen, White Polycarbonate - 389381
  • Eliminates Odor - Exhausts Odors Out The Roof Vent Before They Have A Chance To Invade Your Rv
  • Compatibility - Engineered For A Universal Fit, The 360 Siphon Is Compatible With All Rv Vent Caps

Can I Leave My RV Black Tank Valve Open When Connected to a Sewer?

Absolutely. Positively. NOT!

The black valve can NEVER be left open while using your RV! Full hook-ups or not, the RV black tank valve MUST be kept closed.

Leaving the black valve open would allow the “liquids” to run out of the tank, leaving the “solids” behind to form into a pile (known as “pyramiding”). That can be a major cause of tank odor, and require professional intervention to remove.

Even though you’re connected to the sewer, you’ll need to monitor and empty your black tank as needed. Luckily for us, we only need to dump our black tank about once every two weeks (two people, 45-gallon black tank).

Although we didn’t have pyramiding, we’d like to send out a special thanks to Mike Roe of HydroCleanAZ for super-cleaning our tanks in the video below.

Do You Leave Your Gray Tank Valve Open?

So, do you typically leave your gray tank valve open or closed when you’re connected to full hook-ups at a campground or RV park? Leave a comment below.

And hopefully, you know the answer to the question, “Can you dump RV gray water on the ground?” If you don’t, be sure to check out our post.

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Gary Krejsa

Saturday 18th of May 2024

After I connect to the park's sewer system I leave both tanks closed & monitor how full the tanks get depending on how many days we stay there. I empty black water first and then use the grey water as a so-called "rinse" of my sewer hose. I follow up with a clear water rinse before disconnecting from the park's system. After the valves are closed we dump a couple of buckets of water down the toilet along with a septic tank safe treatment pod to slosh around while we're driving down the road.


Sunday 28th of May 2023

Great information. My neighbors completely cut open/closed valves off and plumbed directly to septic tank thinking it's ok. I've tried to explain how it works but they seem to know everything "not" and after over a year of it being wide open they now have a major problem. I've tried to explain the pyramid of waste just building up but I guess it didn't click in their tiny little brains. So a few days ago I got a question about the smell they are dealing with so I suggested a vacuum truck will be needed to suck tank out because the tank is probably full and the 25 foot of 4" PVC going to septic tank is probably completely full now. 3 kids and 3 adults live in it so that's a lot of waste daily. So their solution was to run a garden hose down the toilet and it would just flush everything out. Once again I warned them not to but they know everything about everything and put the hose in toilet anyway. Maybe 5 minutes later I hear a commotion "turn it off turn it off". Then I hear the f bombs and panic. Now they have both grey and black water tanks completely topped off. I tried to stay out of it but of course I get a knock on the door and I get the dump question about why is it not going down and if I could take a look at it for them. Not trying to sound rude I said " I tried to tell you" and no way in hell I'm looking at anything. How do we get it to drain now they ask. You don't was my response. Call a sewage vacuum truck and have them pump it out like I said before. Everything is PVC glued together so it's going to take a professional company to resolve the situation. They said we just gonna cut it loose and flush it from outside. I had to laugh because of how they don't get it. Do not cut it loose because then you will probably get evicted because where do you think it's going to go then? On the ground and all over you and if you think you have a problem now if you cut it loose I suggest you pack everything up and get ready for some big fines and a very disgusting day. Their solution was was trying to get my shop vac to suck it out from the toilet. Hell no I said. The experts they are got the bright idea to run a hose down the roof vent because it's clogged they said. Where do you think the water will go if. You did that I ask. Oh it should unclog it and it will flow was the reply I got. All I could do was say I'm warning you do not absolutely do not do that. They said it will work. Of course yes they did and in seconds I hear chaos. Stop stop stop!!!! I'm sure you can guess what happened. They flooded it out with the nasty doo doo filled tank water. At this point I went to a friend's house away from the panic. They just ruined a 42foot fifth wheel RV and can't afford a vacuum truck. What do we do now they were talking about. Cry I told them because now your problem you could have avoided just cost you 40 grand you paid for the RV. And now you need to go talk to my mom which is the manager here because that smell is horrible and can't stay like that without everyone complaining. In tears she said her husband is a master plumber and it was the septic tank backed up and caused it. Really? He's a master plumber huh. Wow I hate to see his work on other people's houses or RVs if he mastered this. IDK what to do she says. I said you need to pray because now you are about to PAY. Never ever remove shut off valves for any reason. What a mess. That's a great example of what happens when you leave tanks open. Never never never ever leave gray or black tanks open. They found out the hard way. What a day they had


Saturday 27th of May 2023

You can actually leave your black tank valve open if you are leaned towards the black tank valve just a bit and fill your toilet bowl full full of water before you let if down, along with holding the toilet valve wide open for 15 seconds or so running water down. I do this in south dakota winters with zero problems. I don't recommend it in the summer simply because of sewer gasses get to be alot in the heat and will invade about any coach eventually. I do like the mention of the "whole house" ptrap, never thought of that one for summertime. Just to be clear on the winter thing, I have the hot water line run to the toilet, my holding tanks are bare to the world no skirting. The only hangup you will have if the campground sewer pipe is buried too shallow and it freezes up. Just FYI from my full time living experience.


Friday 26th of May 2023

After thought ???? we close the gray before we shower when it's time to empty the black tank. I think that's enough water to rinse. We've been camping since 1972 and still going for at least another 25 years! ????


Friday 26th of May 2023

We have always left our gray water valve open for at least the last 25 years with no problem or odor

For the black, if you leave it open, the contents will become hard and unremovable.

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