It would seem that if you only switched the two 6v batteries for the old two 12v batteries, you only need to figure out where the four wires in the picture to the 6v batteries need to go. My guess is that one solenoid is to combine the chassis and coach batteries in the event one or the other has been run down. So that solenoid would have wires on each side going to pos terminals of the two banks. Wires from the neg terminal of each bank would go to chassis grounds (bolted to the frame). That leaves the other two in the picture that are connected to the 6v's to figure out where they go. Obviously, if one of the wires connects to the neg side of the 6v's, it's got to be connected to ground.