30 Amp to 15 Amp adapter. Good normal extension cord, and you can run the air conditioner as stated above. But you will not be able to run much more. If you exceed the Amp from the house / garage, the most you'll do is pop the breaker in the house / garage. Nothing hurt. Just reset the breaker, and turn something off in the camper.
Remember, anything on that same circuit (in the house or garage) is adding up to the total 15 or 20 amps that the line is set for based on the circuit breaker. If you have a 20 amp circuit, and 5 amps is running an appliance in the house, you have only 15 you can used for the camper. If the air conditioner takes 10 amps, that leave only 5 left before the breaker will pop. Turn on that hair blower and the breaker my pop. So any electricity being used inside the house needs to be considered also.
The best solution is to eventually have a proper 30 amp plug and breaker installed dedicated for the RV.