Originally Posted by JDKing
these are the 2 chassis batteries. 5.9 Cummins, dsl.
OK -- The OP stated coach batteries.
In regard to automotive batteries, 12 volt batteries are going to be connected in parallel.
The red or POSITIVE cables need to be connected to the + sign on both batteries. The black NEGATIVE cables need to be connected to both the "─" sign on both batteries. There may be a short jumper cable that goes in between both batteries. If the cables are long enough, I would connect the positive cable from the Motorhome to one battery and connect the negative cable to the second battery. It shouldn't make too much of a difference but that's how I would do it. Make sure that the hold down straps are clear from the red or + terminals.
This will provide 12 volts to the starter with increased cranking amperes. If there are smaller gauge wires that are not connected those may need to be connected to the positive cable side however there may be smaller cables that might have been connected to the negative side for purposes of grounding as well.
Don't want to preach but the best thing to do before
you start a project like this is take a picture before you begin.