Better running #8 or even #6 if you have to run new anyway from truck bat to bed plug,@ bed plug butt connect the wire to a size wire that will fit the plug terminal, pick up the other 6 wires along the harness to the rear @ bumper ( 5 + ground), reground at bed plug, run even the elec brake wire , you might get a fifth wheel some day
Like anev942 said use a solinoid if you run new to truck bat , or even if you dont and your trailer harness positve lead is hot with ingition off( you can get one made for this purpose at a rv place) that will cut power btwn camper and truck when ignition is off , just need a ignition on hot wire to power the solinoid . Good idea to put a thermal breaker on the truck end new line if your going to connect to the truck battery
If your running the wire new to the camper battery put a thermal breaker before the camper battery