The single battery under the passenger door step is the one that powers the truck cab, starts the engine, powers the cab heater/ac fan, radio, etc.
The two Chassis batteries power up the coach area. They power the furnace, refrigerator, ceiling lights, roof vent fans, and even power the control circuits for the AC units (the AC units are powered by 110V shore power, but the control circuits for them run on the 12V DC.)
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