Horrible smell from RV toilet - any recommendations?

In our 8th year of full-time and have never used chemicals, just make sure there is some water in tank after dumping and generous water when we flush. We don't flush the tank every time either, just every month or so. All of the items listed above except chemicals need to be checked. The chemical is a deodorizer - covers up the odor, doesn't correct any problem and make it go away. Good luck.

for the past 4 or 5 years, we have been using cheap liquid detergent and Borax, a cup of each in the black tank and the same for the gray tank. we add it each time we dump our tanks, about every 10 days, and have no problems with smells.

I had very good results from installing the 360 Siphon Vent Covers.

Perhaps do not eat as many dishes containing cabbage, eggs, onions, etc, as well as certain varieties of cheese.:E

I run a bleach and Dawn solution twice a year in all three tanks. It kills any bacteria in them and keeps them from smelling bad.

vent video https://www.youtube.com/watch?time_continue=7&v=DsSmGh7XCqw

No matter how clean the tank is it's going to be "dirt" the first time you take a poop or pee. If the tank is not being properly vented from the roof vent you're going to have stink and pressure building in the tank during the day and when you flush it that pressure and stink is going to be relieved into the RV.

Make sure the tube is not obstructed by the vent cover. There should be an inch or so of gap between the top of the tube and the cover. If there is an appropriate gap remove the covering and look down the tube with a flashlight to see if it is obstructed. If there is an obstruction you can pour water down the tube to try and knock loose the obstruction or tie something heavy to a string long enough to reach all the way down the tube to dislodge the obstruction.

The one time we had a problem with black tank odor it was caused by a too narrow space between the top of the vent tube and the tube cover (either the tube or tube cover shifted at some point during our travels). I removed the tube cover, checked for tube obstructions (none) cut 1/2" from the top of the tube, replaced the cover and the issue was resolved.

We have never clean our black tank but we always use a deodorizing packet after each tank dump putting a gallon or two of water back in the tank to prevent whatever is left over in the tank after we dump the tank from hardening. Six years and never an odor issue other than the one time the tank vent was obstructed. That said we did not have sensors in the tank and I understand that cleaning the tank via a black tank flush is something that helps keep the sensors operating properly.

I used to get smells from my grey tank. About once a year it would happen. First I figured traps were dry but that wasn't it. Had a friend tell me he had same issue and the fix was to flush down the roof vent. So about once a year I'd pull the hose up on the roof and blast some water down the two vent pipes. It fixed the problem but still don't know why. I never looked under the sink for an air admittance valve and there should not have been one since there were vents. Just sold that one.

If your tank hasn't been cleaned thoroughly and regularly, it may need a heavy duty cleaning. Our owners manual says to use Liquid Drano. I've used it once or twice a year to get the black tank back to factory fresh, factory clean condition. It won't hurt metal or plastic. (Don't dump it into an on-site septic system.) Some won't use it though for fear of hurting seals. After 4 seasons, no problem for us.

There are commercial tank cleaning chemicals available. I think Thetford has one. It needs to be above a certain temp., then you add some water and drive around to let it slosh around. The "Geo method" is another option (google it for details).

After cleaning the tank by the method of your choice, as mentioned above always put in a few gallons before each use of the tank. Then don't skimp on water each time you flush. When ready to dump, fill the tank to full or near full before pulling the dump valve. If you have an in-tank rinser, let it run 10-15 min. while breaking camp. Or try a back-flush elbow (won't always work depending on tank configuration and outlet location). In-tank rinsers can be added if you have access to the tank.

If your toilet drops down vertically into the tank, you can shut off the water and peer in with a flashlight to see how clean it is. Keeping the tank as clean as you can may also help make the sensors indicate more accurately. I'm a non-believer in tank treaments that you drop in each time you dump the tank. If the tank is kept clean, the AAV hasn't failed and tank vent pipe is functioning right, you shouldn't need treatments. Turning the fan off can help.

A failed air admittance valve under the sink is a common cause of a bad odor. Also, if you haven't used the shower for quite a while, the P-trap could be dry and be letting odors in.

3 dog nights wrote:
... Dawn dish soap. Envision a tank full of bubbles when I arrive.

Don't use THAT much. 1/4 cup or less does the job real good. Won't bubble up and out that way. Yet, enough and strong solution enough to clean everything well.

Another thing you can do. We wash dishes with Dawn and always catch the water in a tub placed in the sink. After washing dishes, I always dump the dirty water down the toilet. That also adds some dishwashing liquid in the black tank. It all helps break down and keep solids from sticking on things and then drying there, which is an odor maker there.