Previous owner would rotate batteries through position 1 (engine), position 2 (house) and position 3 (fridge), new batteries would go into position 1.
Ensure all switches are off on the main breaker as well as switches near the floor on the port side in rear cabin. Connect positive terminals first, making sure to not accidentally allow any of the other wires (or metal wrench) to come into contact with the negative side of battery.
Turn on main breaker in rear cabin to battery 1. Test the battery using main cabin lights circuit. Go back and turn main breaker to battery 2 and re-test. Battery 3 is for fridge, turn this on to check that fridge compressor starts up.