If this can be of any help:
In our Itasca the auxiliary start solenoid (relay) is also the "charge" solenoid (relay).
Whenever the ignition switch is ON, this relay is also engaged so that the engine start battery and the two 12 volt coach batteries are all connected up in parallel with each other. This way the alternator is charging three 12 volt batteries at once whenever the engine is running.
Right next to our auxiliary start solenoid is another solenoid. This 2nd solenoid is the one that connects and disconnects the two coach batteries from the coach system by a switch that is on the wall right next to the entrance door.
