You don't have a disconnect on the chassis batteries unless it was added aftermarket. There is a connection stud mounted on the side of the battery compartment where the positive cable from the chassis battery goes. Corrosion can build up there, even though it may not be visible. Also, loosen the positive cable on the starter and clean up that connection really good, then tighten. Also, as Bob mentioned above, put a more powerful charger on the batteries for a while.
I bought a Schumacher charger/starter from WalMart. When my coach sits a few weeks, it won't start unless I hook up the charger for about a half hour, then use the start function for some additional boost. When the coach is being used frequently, I don't have a problem.
There is a little bit of battery drain when you're parked unless you add a chassis battery disconnect. I bought one, but haven't got around to installing it yet.
USAF, Retired (1962 - 1983)
2000 Itasca Horizon 36LD, 300 Cat
2006 Honda CR-V Toad