Americans often say "20a" when they are actually talking about 15a as in Canada. (same amps different language)
Yes you can run the air conditioner on "15a" (it has a 20a circuit breaker in your Rv but ignore that for this exercise) We have run our 15size a/c on a 15a circuit at campgrounds and at home. But as others have said, nothing else on at the time. Fridge , water heater on gas, converter off or when on the batteries are full.
It is not a killer to have to turn off the air conditioner when you need to run the other things on 120v for a few minutes, then turn them off and turn the a/c back on.
Generally overall, expect to buy more propane to make up for the lack of 30a service.