Model yr of the chassis?
What "power"?
Stop/Turn?
Running lights?
Backup lights?
Brake controller?
Charging line?
Can make a difference..
I am going to assume your looking for charging line power, correct?
Ford puts a relay in line with the charge line, relay only powers the charge line while the ignition is on (IE engine running). They also use separate relays and fuses for the stop/turn, running lights and backup lights for the towing connection.. Prevents shorted trailer wiring from turning off your vehicles lights by accident.
Typically on Trucks the towing relay and fuse box will be under the hood area.. Chassis and van locations may vary..
Newer Fords (2019 and up) now use a solid state "smart" towing module that controls those functions, unsure where that puppy is or the fuses but for the charge line, to get 12V power the engine not only must be running but you must put your foot on the brake and put it into gear.. Supposed to stay on until you turn ignition off and then do that gyration all over.. Not sure what smart idiot came up with that gyration but it is a stupid one for sure..