Wiring relays

Run a large circuit breaker off the positive battery post to a constant duty solenoid (type of relay). The solenoid can be switched on and off by your ignition circuit or a dedicated switch. From the output for the solenoid, run a heavy gauge wire to the rear of your truck and at the rear run a second heavy wire off the frame for ground. Using an Anderson connector, you will now have a high amperage power point that can be plugged in to your trailer for better charging, power a receiver mounted winch or even be used as connection for jump starting other vehicles.

Since you plan to to connect to your trailer batteries, use the same type of heavy circuit breaker from the positive camper battery to heavy gauge wire and second heavy gauge wire connected to the trailer frame for ground into the same type of Anderson connector at the trailer tongue.