Your description reminds me of my years ago experience with a prior RV, a Bluebird Wanderlodge. When driving down the road, if someone used the loo, then after flushing, the black tank smell would get sucked up into the bathroom. As soon as the bathroom door was opened... well, you can guess what happened. My point is that if you have negative pressure inside your coach, then you could be getting a low-level smell coming from either a toilet that isn't setting tightly to the floor or the ball seal that isn't sealing properly. Also, in our current Beaver Marquis, we have smelled waste tank odors from the washer/dryer area due to the washer's drain trap drying out and not making a seal against returning odors coming up through the piping from the waste tank.
In the case of the washer's drain trap drying out, the smell was only barely perceptible when we walked by the washer. The smell could easily have been mistaken for a leaking propane smell. By the way, our fridge is located immediately across the aisle from the washer/dryer.