It depends on your RV. Generally we see RV’s that use either 15,30, or 50 Amp power. Here’s a breakdown of extension cords for each power level:
These are the smallest of them all. They run on a power that is identical to the power you get from the outlets at your house. For RV’s that need to get 15A power, we recommend using a 10 Gauge Extension Cord. This is a good cord to run at any length, as it has a very durable jacket, and thick stranded wire. At minimum we recommend using a 12 gauge extension cord.
This seems to be the most common size RV and power on the market. Extension cords for 30A RV’s are rated for 30 Amps at 125V. Almost all of the cables on the market will be 10 Gauge. You will have no problem running 30 Amps though these cords as well.
These are the big boys of the RV world. They run on hefty 50A, 125/250V power. And with big power comes big…gauge. You’ll want a 50A rv extension cord with 6/3 + 8/1 or 6/4 gauge wire. It is more common to find a 6/3 + 8/1 cord on the market.