Many times this takes some trial and error with a digital mult meter to see which wires read 12v when the van is on but 0v when its off. Using a smart battery isolator like the ctek d250s there is no need to use expensive and bulky 4 awg wire as 10 or 12 awg wire is sufficient for battery charging.

Noco Battery Isolator Manual High School
How to install a battery isolator. The isolator needs to be mounted to the actual metal frame of the vehicle so make sure you can press it flush to the frame. Step 3 wire the isolator. Pick a place that puts it between the engine battery and the house batterybut make sure it is easily accessible should you need to perform any maintenance later. A battery isolator typically is used in situations where multiple batteries are required and its primary purpose is to ensure that the failure of a single battery will not incapacitate an entire electrical system. A battery isolator is a one way electrical battery that allows an electrical current to flow in one direction but not the other. Step 2 install isolator.
For battery isolation you want your relay to activate when the van is running. This is an extremely important step so that you do not. The best way to do this is to find the accessory wire in your fuse box and tap into that wire. Begin by making sure that the car is turned off. The expedition portal wire diagram i designed the portable dual battery system for my ford e150 camping van and asked an electrical engineer friend to piece it together install it. Decide where you will want to mount the battery isolator.
Wiring a battery isolator. Step 1 turn everything off and prepare.