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, regular RV gray water tank maintenance should address the issue. That’s why, in this post, we discuss properly maintaining your grey tank, along with our top tips for preventing foul odors 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 often black plastic, 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 your RV’s sinks, indoor shower, washing machine, and dishwasher. 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 any products you use (such as dish soap, hand soap, body wash, shampoo, and toothpaste), along with body oils, 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 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 more pungent 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 may lead to clogged or blocked tank sensors. 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) when we’re boondocking, we use a paper towel (or our used napkins) to wipe food particles out of any pots or pans, as well as off the dishes, BEFOR washing them.
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 up. 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 half 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. Plus that rush of gray water leaving the tank helps to remove any particulate matter and residue left on the bottom of the tank.
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 (such as Happy Camper Tank Treatment , Unique RV Digest-It , or the Geo Method) to help keep the tank as clean and odor-free 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
- Eliminates RV Tank Odors Fast: Our mineral-based formula tackles foul smells in RV, camper, and marine holding tanks. Prevents lingering waste odors...
Happy Camper Extreme
- Super clean tanks
Unique Clean-It (lists itself for black tanks, but it works equally well to clear stubborn gray tank odors)
- 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.
Tankoscopy!
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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Kathy Kenler
Sunday 22nd of June 2025
I find your newsletter very informative.. I do have a problem and am hoping you may have suggestions for me. When I flush my rv toilet there is a sewer smell that develops outside, no smell inside just outside, the black and grey valves are closed and can’t see any leaks, what might be the cause of this?
Kathy Kenler
Wednesday 25th of June 2025
@TheRVgeeks, the smell comes from underneath, where the black tank is, and the winds seems to push it out into our campsite.
Kathy Kenler
Monday 23rd of June 2025
@TheRVgeeks, thanks for your reply, it seems to be when the gage is reading 2/3 and over when the smell happens.
TheRVgeeks
Sunday 22nd of June 2025
Hi Kathy! The most likely thing we can think of is the roof vent, since that's a direct outlet from the black tank to the air outside. We'd be curious to know if this happens when the black tank is completely, or mostly, empty, or just when it's more full? It would be more likely to be coming from the black tank rather than the toilet itself.
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!
Kathy Kenler
Wednesday 25th of June 2025
@Gay Tacoma (Washington) Travel Enthusiast,