Everthing thats electrical, radio, tv, monitor etc has the potential to draw small amounts of power even when off. Plus the truck has its own parasitic drains. Im assuming this camper doesnt have a battery dissconnect between camper & camper battery?
Yes if the camper is tied to the truck as described you need to install some type of isolater-not only when parked but camping you'll be pulling off the truck. The truck should charge camper while running but be isolated when not running. Simplest and least expensive is a constant duty solenoid-any rv or automotive parts house. "Constant duty" solenoid. You need to address (or unplug camper), repeated draining of your starting batteries not a good thing
The light wiring issue I believe is seperate (not causing drain) but a lot of possilbilites. One being there is a short in camper sig/brake wire-which CW should fix. Could be as simple as a stray wire strand in one of the umbilical plugs, corroded light socket. Verify 1st its not the truck wiring to the plug.
