[JRTJH]

The ROMEX size is the issue with changing out a 15 amp breaker (normally 14 ga ROMEX) for a 20 amp breaker (normally 12 ga ROMEX).

If you're sure that you have 12 ga ROMEX at the circuit breaker AND 12 ga ROMEX at the air conditioner "rooftop connection" and that there's nothing else on that circuit (in other words, no junction boxes where 14 ga ROMEX might be tied into the line) then changing out the breaker "might be safe".... If you're not knowledgeable enough about the system and wiring to make that "very specific determination" (that there is absolutely NO 14 ga ROMEX) then I wouldn't suggest changing the circuit breaker.

That said, there is "more than 15 amps" being drawn by whatever is connected to that breaker, so determining WHY it's breaking the current is more important (in understanding what is happening) than "fixing it with a larger breaker" which may cause overheated ROMEX and possibly a fire.

If you're not 100% sure of your electrical skills, I wouldn't change out anything. This is one that's best left to someone who understands the wiring codes and can relate them to what is happening in your trailer.