The easiest way to accomplish what you are asking is to wire the starting battery to the 2 coach batteries with a switch in-between them to dis-connect when you un-plug the motor home. All you need to do is add a wire from the positive terminal of starting battery to the positve terminal of one of the 2 chassis btteries with any type of switch in-between the batteries.
There is of course better ways involving using electronic solenoids or relays operated when you start or turn off engine. You could also add isolator in-line between batteries, but that will take a lot more wiring and money too.
The factory on/off switch is only connecting/dis-connecting that bank of batteries to the coach. In order to charge both banks of batteries you need to physically connect them somehow. The thing you want to be aware of is leaving the batteries all hooked together when not plugged into 120 volt outlet will cause drain on all of them and if used this way too long could prevent the engine from starting. You would want to make it like the old school campers with a "dry-camping" position and a "shore power position on switch.
That way on shore it is always charging both banks of batteries and on dry you will be seperate.