Steve-
When traveling, sometimes liquid gets sloshed out of P-traps (sinks, shower, washer). If one of those P-traps is infrequently used, and has dried out, gases can migrate back up into the coach through the open drain; a cup or two of water added to our shower drain stops the problem for us, as we don't necessarily shower every day ( ??)), especially when dry camping and trying to conserve.
Our shower drain is the only one that doesn't have an AutoVent, and is the only one that ever causes an odor problem. The AutoVent is a valve above trap level connected with a "T" in the drain line between a trap and the tank, and it is normally closed. If you have AutoVents, you should easily see them under your sinks. When you dump your gray tank, the AutoVent is opened by the vacuum caused by the tank drawdown, and this prevents the P-trap from being sucked dry each time you dump your tank. I have to remember to pour a cup of water in the shower drain after each tank dump, because, with no (apparent) AutoVent in that line, the resulting vacuum empties it, and we can smell it within an hour. AutoVents can conceivably get stuck open, allowing tank gases into the coach; I've never had that problem, so have never attempted to check if one was stuck open. If that was your problem, though, you'd have the smell parked or traveling. I still need to carefully check our bays to see if there is indeed a shower AutoVent hiding under the floor; if there is one, it may be stuck in the closed position.
Leaving the bath vent open while on the road is not something we do, as like yours, the tank vents are next to it, though forward not aft; in certain circumstances you can actually cause negative pressure inside the coach, which will draw fumes in. Many vent mfrs. advise against travel with them open, for various reasons.
-Joel