Black tank smell in coach

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.