I have a 5er with a 5500 Onan generator that I installed, and is working great. Since it is going so well my camping buddy is now doing the same. He is installing a 4000 Onan, but it is not going as well. The battery location in his 5er is in the rear and only one 12 volt. He does not want to string wires from the front belly to the rear of the 5er. Right now he is using a second battery located near the generator to start it, but it is not running through the campers charging system. How can we run this new second battery through the campers converter/charging system? ???