It's back....and I'm stumped

Did you make sure your black tank vent on the roof is not clogged?

Look under the bath sink for a vacuum check above the P trap. If it is weak, fumes can be sucked into rig.
Air admittance vent

Recently, each time we drove the MH, after about 10 miles we'd get a strong sewer smell also. Figured out it had to be one of the sink drain line vents that weren't venting properly. After doing some reading, the mfgs recommend changing them out every ten year - our were about eleven years old. Changed all three since I wasn't sure which one was the problem. No more odders.

I first went to Home Depot and bought three of them. Went back to the MH and got two of them installed. On the third one, it was right up against the sink and the ones from HD were too large of a diameter to install. It took a while, but I finally found some at an RV place that had the same small diameter as the original ones.

Look in the picture just to the right of the sink at the top of the black plastic pipe - that's your vent valve. Just unscrew.



Bill

I had this too and it turned out to be the slide seal was leaking ,yet it would hold water. don,t no how ,but changed the slide seal and it was fixed.

To those suggesting it's the air admittance valve, I stated in my original post that it is brand new and functioning correctly. There is NO oder under coming from under the sink.

I have not yet checked the vent on the roof. Just looking at the physics of this, it seems that if it was clogged, air couldn't get in. If air can't get in through the vent on the roof, how could my fantastic fan pull air into the RV from the black/grey tanks? You can't pull air through a straw if your finger is on the other end. :/

Correct, if the vent is clogged air can not get in or out of the vent.
If the vent is not clogged, as you drive the air will be pulled out of the black tank.

You notice the smell while driving or, when the fan is running.
Possibly something is blocking the vent that acts as a one way valve? Air can get in the vent but, not out.

Driving or running the fan causes a slight vacuum in the tank and the smell is possibly seeping out around the rubber seal on the toilet. Try resetting or replacing the rubber seal, maybe it's not seated correctly?
But I would start with checking the roof vent.

That's all the physics I can offer. :)

I would close all the doors, windows & vents and when the odor is strongest, start the smell test and follow it and at least try and narrow the location down.

What kind of motor home is this? If you put it in your profile, it makes it easier to diagnose problems. Is your coach a single bath or bath and half? Is the toilet located directly over the tank?

Often, if your toilet is right over the tank and water bay, you can get odors coming up from the water bay through plumbing cutouts behind the toilet. Often this area isn't completely sealed.

what model thetford toilet?...on the Aqua magic V, the seal for the flush handle wears and starts to stink.

What type of vent cover do you have on the roof? Depending on how it is installed, or what is near it, it can be forcing air DOWN the vent pipe as you drive instead of sucking the fumes out of the black tank.