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Can GRAY Water Smell as Bad as BLACK? Gray Tank Maintenance 101

Can GRAY Water Smell as Bad as BLACK? Gray Tank Maintenance 101

When RVers are hit with an extremely foul smell coming from somewhere inside their RV, most people immediately rush to the RV toilet or black tank. But as it turns out, your RV’s gray tank can smell just as bad (if not worse). Fortunately, some good RV gray water tank maintenance should take care of the problem.

In this post, we’re talking about maintaining your RV’s grey water tank and giving you all of our best tips for preventing a bad odor from invading your rig.

What Is an RV Gray Water Tank?

Your RV gray water tank (or “grey water tank” if you prefer) is a holding tank generally mounted under your RV, and sometimes colored gray to identify it as the gray water tank (RV black water tanks are usually black, and most RV fresh water tanks are white). Combined, all of the gray water drains, plumbing, and tank(s) make up your RV’s gray water system.

Gray water is comprised of everything that goes down the drains of RV sinks, indoor showers, washing machines, and dishwashers. This means that gray water contains water plus any products you use in the shower, in your bathroom and kitchen sinks, and in your washing machine or dishwasher (if your RV has them).

Why Does Gray Water Stink?

Again, many people aren’t aware that gray water can smell as bad as (or worse than) black water. But “gray water” (or “grey water”) is water mixed with a whole lot of other stuff… namely anything that runs down the drains of your sinks and indoor shower. This includes products used and washed down your sinks and your shower (dish soap, hand soap, body oils, toothpaste, etc), saliva, and food particles. Ultimately, the often terrible smell of gray water is due to the build-up of organic matter, fat deposits, and bacteria in the tank. 

Boondockers also have what’s called “concentrated” gray water. That’s because extreme water conservation (read our recent post on tips for minimizing the amount of water you put in your gray tank) leads to a higher concentration of food particles, body oils, soap, toothpaste, and other residue in relation to the amount of water in the gray tank… which can lead to a much stronger odor.

John near the RV kitchen sink indicating a foul odor

We once had an issue with an air admittance valve in our Newmar Mountain Aire that resulted in a terrible odor coming up through the p-trap from the gray tank.

RV Gray Water Tank Maintenance to Prevent Foul Odors 

Regular use and regular dumping should generally keep your RV’s gray tank operating fairly well. It’s also helpful to try to avoid washing too many food particles down the drain, as you’re just adding organic matter that can be the source of stronger odors and can lead to clogged/blocked tank sensors. When we’re boondocking, we use a paper towel (or our used napkins) to wipe food particles out of pots and pans and off of dishes before washing them, to help minimize the amount of food being washed into the gray tank (and to reduce the amount of water needed to wash them, a double win). 

Also, some RVs (though very few of them) have a sani-flush system on the gray tank, similar to the black tank flush. We’ve never had one on any of our rigs, but if we did, we’d sure use it each time we dumped the gray tank to keep our tank sensors in good working order.

Following are some things you can do to properly maintain your RV’s gray tank and prevent an excessive buildup of slime/sludge (and odor) on the bottom of the tank:

Close the Gray Valve

Leave the gray valve closed even when you’re fully hooked. This way, when it’s time to dump the tank, a large volume of water will rush out and help pull residue from the bottom of the tank out with it.

PRO TIP: if you’re at a full hook-up site and want to leave the gray valve open so you can enjoy unlimited water use, just be sure to close it the day before you plan to dump your black tank. Having at least 1/4 to 1/2 of a tank of gray water will allow you to “rinse” your sewer hose after dumping your black tank, just by opening the gray tank valve and allowing the accumulated water to do its job.

More Water and More Dumping

Intentionally use more water (to increase the ratio of water to waste) and dump more regularly (to keep sludge from building up in the tank).

Use a Tank Cleaning Product

Use a tank cleaning and odor control product like Happy Camper Tank Treatment or the Geo Method for RV tanks to help keep the tank as clean and clear of odors as possible. We discuss these products further in our post on RV black tank cleaners. But tank cleaning products like Happy Camper, Happy Camper Extreme, or Unique Clean-It can also be used in your gray water tank to digest/remove waste material from the tank.

Happy Camper Tank Treatment

Happy Camper Extreme

Happy Campers Extreme RV Holding Tank Cleaner
  • Super cleans RV holding tanks
  • Sensors: Restore poor working sensors

Unique Clean-It

Unique Clean-It RV Black Tank Cleaner Liquid, Enzyme Deep Cleaner Digests Waste Accumulation and Buildup, Eliminates Odor, Formerly Tank Cleaner (32 oz.)
  • New and improved formula! Using Clean-It is like power-washing your black tank. Works in 48-72 hours, no driving required. Cleans more effectively...
  • We took Unique Tank Cleaner, the strongest RV black tank cleaner on the market, and made it even stronger! Clean-It is powered by billions of enzymes...

Perform a Tank Soak

Do an occasional “soak” of the gray tank by adding any one of the following into the tank to help clean it out:

  • Automatic dishwasher detergent (1 cup)
  • Dawn dish soap (½ – 1 cup)
  • Borax/Calgon water softener (1 cup dissolved in 1 gallon water)
  • Laundry detergent (1 cup)
  • Baking soda (1 cup – dissolved in 1 gallon water) 

Add any of the above to the tank and either fill it the rest of the way with water and let it soak overnight or get it about half full and take it for a drive. The sloshing helps to agitate the solution and clean the tank.


If things are really bad, you may need to resort to having your tanks professionally pressure washed. Even with good maintenance, a mineral called struvite can build up in RV holding tanks over time (and depending on the mineral content of the water and other materials in the tanks). So, if you have an older RV, a bad gray tank odor, or a tank sensor issue that you can’t resolve with any of the less-invasive techniques noted above, seek the assistance of a professional tank washer. We kept our holding tanks super clean… but, even so, after many years of use we had to do it!

Have you ever had a bad situation with gray tank odor? Or have a different technique for helping clean/maintain your gray water tank? Drop us a comment below, we’d love to hear about it.

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Gay Tacoma (Washington) Travel Enthusiast

Tuesday 11th of June 2024

I love the expression on Pete's and John's faces. Peter looks ready to spew from the stench, and John's no different! Ew!

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PLEASE NOTE: We're handy RVers, not professional technicians. We're happy with the techniques and products we use, but be sure to confirm that all methods and materials you use are compatible with your equipment and abilities. Regardless of what we recommend, consult a professional if you're unsure about working on your RV. Any task you perform or product you purchase based on any information we provide is strictly at your own risk.

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