My old camper had a crank out window in the living room and we installed a vent cover in the bathroom. Left the window and vent open year round, it was fine. No smells, no mildew stink, no black tank smell.
New camper is under a carport cover, so we leave the bathroom vent open and windows on the slideout open year round. Again, no smells and no issues.
I wouldn't run the AC unless I was worried about humidity. Even then, its probably cheaper to run a dehumidifier that drains into your largest holding tank (or run a hose to the outside). Keeping your RV cool when you don't use it is just a waste of money and electricity.