Pick the right amp rating first, then worry about length.

Your RV’s electrical system is built for either 30A or 50A—match the cord to that. A 30A cord on a 50A RV won’t work, and a 50A cord on a 30A RV is overkill but fine.

Length matters for voltage drop. For a 30A RV, a 25-foot cord is standard; go to 50 feet if you need it, but no longer unless you step up to a thicker gauge (10 AWG for 25 ft, 8 AWG for 50 ft). For a 50A RV, 50 feet at 6 AWG is about the max you’d want. Past that, voltage drop can mess with your air conditioner or converter.

Buy a cord designed for RV use—not a cheap extension cord from the hardware store. RV cords have heavier insulation and locking connectors that won’t wiggle loose. A good one costs $50–$150 and lasts years.

Don’t save $20 by buying the wrong cord—you’ll just end up with tripped breakers and fried electronics.