The coach batteries will probably not be directly connected to the chassis batteries, by a heavy cable . They will be attached to the chassis charging system, through relays or an isolator to prevent the coach from draining the chassis ( engine start ) battery while your living in the coach.
Reversing the connections on the coach battery , then connecting to shore power or starting the engine , can result in damage to the convertor/charger, alternator on the engine ( if not properly fused ) and any circuit boards in the coach,( fridge, furnace , air conditioner) so getting the connection right the first time is important.
I take it from your question the coach battery cables are off and not clearly marked. You will need a multi meter to test before you attach any wiring.
Test the connections , with the meter set to the ohms scale, with the meter grounded to the vehicle frame , the ground wire will show zero , ohms.
The battery power wiring should show an open circuit. Then with the cables still disconnected from the coach battery , and the multi meter set to DC volts, start the engine , the wiring that showed open circuit , should now show charging system voltage, 13.2>14.2 volts.
With the engine off, plug the coach into shore power , the same wire you found charging system power from the engine will have voltage from the convertor charger.
You can operate 12volt lighting, in the coach with the cables reversed , BUT any other 12 volt system in the coach can be damaged by reversed polarity.