Well, maybe. But in 12 years and two rigs, I have NEVER had the slightest bathroom odor. And I do not use chemicals of any kind. If you were really forcing water through a trap or the water in the toilet, you would surely hear it. (Maybe you do?).
I don't mean to be argumentative, and you obviously know your rig and I don't. But I think you have a leak, or else a design flaw in your plumbing.
Formerly: 2005 Airstream Interstate (Sprinter 2500 T1N)
Now!: 2014 Great West Vans Legend SE (Sprinter 3500 NCV3 I4)