A 20 amp circuit breaker is 20 X 120 = 2400 watts. This is more than enough to run an RV A/C.
You are probably running something else that is high current and turns "on" and "off" automatically. An electric water heater does that. An electric coffee maker could do that.
Switch the water heater circuit breaker "off". See if the A/C works better.
You may wish to read the generator operators manual and see if you are using the generator correctly. 3600 watts should be enough to run both A/C and water heater. However, a 20 amp breaker says you only have 2400 watts.
I wish you good luck and happy trails ahead!