Black tank smell in coach

Check under the sinks. There is a check valve that can develop a brittle rubber gasket. Replacements are under $10 at box stores.

What's Next? wrote:
I have had our coach for 7 years now and every once in a while while driving I get a really bad smell coming into the coach. It smells like the black tank is venting into the coach. The weird thing is it doest smell bad in the bathroom. I always flush out the tanks really well and add chemicals to the black and gray tanks. Any help is appreciated.

Use a lot of water and dump often and in which case, chemicals aren't needed. Like 4 days tops with two people. Most likely it's air down drafting through the vent pipe and in stubborn cases, it's often been found that the vent pipe was installed all the way to the bottom of the tank, rather than near the top. Some have installed electric exhaust fans on the roof, but not sure of the results.

Agree with Wolfe10. Link below has a picture of what you're looking for.

Studor or air admittance valve

It will be on top of a vertical pipe near the sink plumbing. Buy a spare as they are cheap.

Fred

Does your coach sit a long time in between trips? I had the bathroom sink p-trap dry out and whew it let in black tank smell. Took me awhile before I figured that one out!

p-traps can dry out letting smell into the coach. Especially in warm weather states. When I traveled to Florida that bathroom sink trap would dry out real quick.

Also sometimes the gray tank can smell just as bad as the black. Check all the sinks AND the shower and pour water down them to fill the p-traps.

What's Next?
I used to have the same issue with our coach when we first purchased it. If you have a washer/dryer in your coach (like we do) you may discover the smell is coming from the p-trap for the appliance. The W/D is probably the least used appliance/plumbing fixture in the coach so I make a point of running a wash cycle through it when we we're set up at a park. If your coach sits for a long period (especially in CA) chances are the water in that p-trap has evaporated - we're in Orange Co. This resolved the problem for us - hope this helps.
Safe travels,
Paddy Drumm

If the smell is THAT bad, then before you start checking all those valves, traps, and seals -- nuke the gray & black tanks with bleach.

Like already posted, I'd bet on a combination of the vent pipe pushing air back into the tank and into the coach as well as dry sink/drain traps. Even if your traps aren't dry when you start out, the sloshing on going down the road can cause what little water is in the trap to drain out and leave a clear shot from the holding tank into the coach. Remedy? Make sure your tanks are really clean and flushed with copious amounts of water. And add in RV tank enzymes in the black tanks to digest all the waste so it flushes out good. Seems to work really well for mine. When we first got our coach, we smelled the sewer when going down the road. After a thorough cleaning/flushing of the tanks and putting a bottle of the enzymes in after each dump, we have had no more problems.

I have never done anything to my grey water tank in any motorhome. Do you folks treat it or flush it. I would be interested in the maintenance you do do to grey tank. I flushed my black tank frequently and have done all the the things like calgon and ice cubes etc. I now have an external flush valve which is nice.

I was driving through Ohio and had a really bad smell coming in. At first I thought it was the area, but it lasted too long - well over an hour. Then I thought there might be some thing on my shoes but that turned out wrong. What was happening was I was about a half mile behind a hog truck.

We had the same problem and the air admittance vent under the kitchen sink was the culprit. Went to home depot and replaced it and the one under the bathroom sink and no more "Who farted?" looks from the wife:B