Go with 10-gauge for a 50-foot run.

10-gauge. At 15 amps and 50 feet, 12-gauge is technically fine for short bursts, but your RV air conditioner will be running for hours. Voltage drop on 12-gauge at that length means the compressor works harder, draws more current, and runs hotter. That’s how you melt a plug or trip a breaker on a hot day.

10-gauge keeps voltage drop under 3% at 50 feet. It also leaves a little headroom if the AC surges on startup—which it will. And if you ever want to run anything else on the same cord (like a microwave), 10-gauge won’t force you to choose.

Don’t skimp here. A proper 10/3 outdoor extension cord (with a NEMA 5-15P and 5-15R) runs about a buck a foot. That’s fifty bucks to not worry about your RV catching fire or your AC short-cycling. Worth it.