Your NORTHSTAR Arrow should have a voltage sensing relay already installed, up near the back of the fuse panel, it is a small plastic cased rectangular box with wires going into the bottom. Ours is Gray about 4"x 4", when the camper batteries go below the low voltage limit,
I think ours is 12.7 volts, the power connection to the truck will be
disconnected. The relay closes at 13.7 from the truck to begin the camper battery charge cycle.
If you open the door under the sink, look up on the panel area behind that fuse panel.
Ours is made by BEP Marine.