Exactly what he said........ get yourself a 30A to 15A adapter and as long as you are just running the garage A/C, you will be fine since it is on its own 15A breaker. That way you don't have to leave a door or window cracked for the extension cord.
Also, I had a 2018 29H and installed solar. I was rewired my inverter and was able to run the garage A/C off the inverter on a sunny day without any issues. I did upgrade my inverter to a true sine wave inverter however, which is better for an A/C unit.