Some motorhomes have an auxiliary switch in the driver's cabin. When your house batteries are dead, you press the switch and it ties the house and engine batteries, and allows you to start the generator without having to start the engine. If yours does not have the auxiliary switch, you can start the engine, but it will slowly charge the house batteries. Not all motorhomes allow the engine to charge the house batteries unless they are equipped with a BIRD device. This device will also allow the generator to charge both house and engine batteries.