Pete,
the biggest problem is not the extension cord, but the wire in the ceiling and walls that runs from the breakers to the A/C. That must be rated at 30 amps. NFPA code (national electrical code) calls for 30amp branch circuits (what you are doing) to be wired with 10-ga wire. If you use 10ga, then you
can fuse (or use a breaker) at 30 amps. that way, the circuit from the breaker panel to the ac would be ok even though you might smoke that 15amp extension cord (probably 14ga.). At least the extension cord would be smoking *outside* the trailer and could be replaced at your leisure.......

thanks,
denny