safe bet that your air admittance valve under your bath sink is leaking or loose.
Could be empty traps, but seems to be common that the factory doesn't tighten these thing sup. i found both of mine to be less than hand tight. We had the smell too. Odd thing was that it wasn't all the time or even on every trip.
Anyway, I found one of them to leak when I took it off, inverted it, and put some water in... so I found a hardware store near the CG we were at at the time and just bought two new ones.
I think we may have had a time or two when we had a minor odor problem form the tanks but nothing like before....more likely just from during the flush, etc...or maybe when i don't get a full dump... I've taken to keeping a small bottle of the blue stuff on hand, and if it gets to smelling bad, pour a little in and it knocks the odor down very well untill we get to a dump station... just need it on occasion
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