Use your volt meter or a test light. It may not be the centre pin, on some vehicles, the centre pin is for the backup lights. You can find trailer connector wiring diagram at etrailer.com. Also on Ford and some other vehicles, a fuse or relay must be installed in the fuse block under the hood to send power to the trailer connector.
Ken
2006 Winnebago Outlook 29B E-450. 2012 Honda CR-V AWD Blue Ox Aventa LX tow bar and Brake Buddy Vantage.
The center pin is the trailer backup light circtuit. You want the top-right pin.
Ford used to ship trucks and vans with the relay for the pin un-installed, but I think RV's typically come with it installed. Wouldn't hurt to check if it's missing power. And yes, it's Key-switched.
2000 Ford E450 V10 VAN! 450,000+ miles 2014 ORV really big trailer 2015 Ford Focus ST