The starter relay and the relay that connects the house and engine batteries are 2 different solenoids. The engine battery is used to energize the starter when the key is turned. At some point after the engine is running, the other solenoid is energized so that the alternator can charge the house batteries. The same solenoid is used when the "battery boost" button is pressed to use the house batteries to start the engine. In some coaches, that solenoid will also provide a way for the converter to charge the engine battery while parked on shore power. That last configuration is not available on all coaches.
