Your Ford does not seem to have the relay that only lets power to the trailer plug when the ignition is on. Chevs are like that, but they have a fuse you can pull.
If your truck does not have a fuse like that, then you could snip the power wire that goes into the camper's plug (pin #4) which will mean no alternator charging, but no connection between camper "house" 12v when parked. The camper tail lights etc would still work.
Or perhaps you could put one of those relays in the Ford like newer Fords have if yours doesn't have one.