My 15K BTU will run about 15-20 minutes on a 15 amp circuit before thermally popping the breaker it is plugged into. That is also feeding th loads on the converter-charger in the RV and whatever lights are on in the garage.
So you can probably plug into a 15 or 20 amp circuit long enough to check things out, including "is the A/C working?" but maybe not long enough to cool it down or live in it in hot weather.
If the circuit you want to use is GFCI protected, things can get more problematic, not the least because of the second GFCI in the RV, as well as wiring faults.