I drilled a hole on the inside of my bed big enough for the cables and put a rubber grommet in the hole, then fed the cable through. I used a set of jumper cables because it was the cheapest way to get 25 feet of heavy gauge two conductor cables.
Protecting from chafing is mostly a matter of securing the cable really well so none of it moves, but there were a couple places I put some large rubber hose around the cables.
A circuit breaker really is a good idea, these are big cables connected straight to the batteries running most of the length of the truck. A short would rain on the party.
The truck will charge the camper batteries. Once they are connected to the truck batteries, the alternator doesn’t know or care where the batteries are. The low batteries will put a load on the alternator and it will accommodate it.
