Absolutely no power

I have a similar problem on my 2018 Thor Chateau C class. Had TWO batteries go dead on a recent trip through the Northwest. Could not take time to mess with charging so wound up replacing chassis battery twice. Obviously a parasitic drain but can't figure where. Have found that turning off the main house battery disconnect switch (next to the entry step) seems to work. I only turn on that switch when I need 12 volt power or are actually driving the coach. When parked and hooked up to shore power I turn the switch off, everything still works in the coach. Coach going in for warranty work next month, finding this problem is at the top of the list. Did call a Ford rep and he mentioned that a "transfer switch" in the engine compartment might not be working properly, hard to accept on a brand new unit.