In your battery area, you should find the ground/negative wire connected to the frame, which ever color that may be, so that is the one that goes to the negative terminals of the batteries. The positive cable will attach to the 12 volt breakers on the wall behind the batteries. When connecting your batteries, the positives connect together and the negatives connect together. Then, connect the negative/ground cable to the negative terminal of one of the batteries and then connect the positive cable to the positive terminal of the other battery. This is a photo of our connections, the ground cable connected to the frame with a copper lug, and the positive cable connected to the 12 volt breakers. In our rig, they used white for the negative cable and black for the positive.
Bob & Becky
2012 Chevy 2500HD D/A CC 4WD