The AC doesn't put a vacuum in the coach, so I'm not sure that the AC on or off makes a difference. It's probably just moving the air around so you can smell it. Another check would be to close all the windows and doors and turn on a vent fan. This would put a vacuum on the coach and if an AAV or P trap is the problem it will be obvious.
Most RV toilets are a straight shot to the black tank. If your bowl seal is leaking (water doesn't remain in the bowl) then that's another source. Also, you need to let the flush handle snap closed (some types you can lift up for more water) in order to get a good seal. My DW has a thing about making noise in the bathroom and her gentile method of flushing usually results in no water in the bowl and a stink.