Horrible smell from RV toilet - any recommendations?

nayther wrote:
I never looked under the sink for an air admittance valve and there should not have been one since there were vents. Just sold that one.

Without an AAV under the counter by the sink, the water would be siphoned out of the sink's trap EVERY TIME you ran more than a trickle of water. The AAV is to break the vacuum that the flowing water otherwise would generate so the water stays in the trap.