What gauge extension cord should I use for a 50-foot run to an RV air conditioner?

Go with 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 makes the compressor work harder, draw more current, and run hotter—risking a melted plug or tripped breaker. 10-gauge keeps voltage drop under 3%, handles startup surges, and lets you run other devices on the same cord. Don’t skimp here.