My F450 came with factory installed 7 pin trailer wiring and plug. As per standard wiring for 7 pin connectors, one pin is 12V for trailer power and for charging the trailer batteries while underway. This is a 30 amp circuit, and has a fuse position in the engine compartment relay box for this fuse, and it is labeled as such. But, Ford did not put a fuse in, so to get power I had to simply plug in a 30 amp fuse.
So if you have a factory installed 7 wire plug, look for a missing fuse. Otherwise, do some wiring, don't forget to fuse it!!!
Funny story, when I picked up my new 34 ft. gooseneck horse trailer, I did not check the battery wiring (it has two large house batteries for the living quarters.)
A few days after the 1000 mile round trip, I was wringing out the wiring, no power from the truck. Fuse blown. Hmm. Checked trailer wiring, then went to the battery compartment. Dummies at the trailer builder wired the batteries positive ground!!!!! Good thing the truck supply was fused, otherwise I would have blown the alternator.