Two questions for you:
1) Was the engine running when you checked for 12V at the hitch connector, and
2) Has the 12V charge line ever worked and stopped working, or (as far as you know) it has never worked?
Ford does not provide a 12V charge circuit in the V-10-based F-53 chassis, such as yours.
For years Newmar has provided wires between front and rear for this purpose, but has not connected them at either end.
If the charge line has never worked, then the instructions in this post
by "JerryP" will help you get those extra wires hooked up. Or, they could help you find what someone else hooked up, and possibly the fuse that serves it.
For more information, see this thread:
What and Where are extra wires from rear?