The 2 motorcycle batteries are for the EAS system, I think to be able to shift it into neutral with engine off. Much of the electronics is 24V but for the non EAS versions the convertor in the cab is enough.
I have 5 more batteries in mine - 4 4D Lifeline house batteries plus one yellow optima to start the generator. But I can interconnect them with switches which gives me redundancy as far as starting is concerned, also if my battery charger dies I can charge the camper straight from the alternator.
Charlie