For a 7-Plug RV, the center pin is most commonly used for Back Up lighting. If your vehicle was equipped with the Factory tow package, you should have two relays under the hood in a smaller box. One relay is for the trailer tow charge, the second is for the back up/AUX. With the key off, you shouldn't be able to measure any voltage at the plug. Turn the key to ON, and it will engage the Trailer tow relay, then there will be voltage at the back.

DarkBlue Trailer Electric brakes
Orange Trailer Tow Charge
Yellow Left Turn
Dark Green Right Turn
White Ground
Brown/White Trailer Running Lamps
Black/LT Green Trailer Back up lamps

Again, this only applys if you have the factory tow package.