If your house/coach battery is hooked up to the alternator to charge while driving, the isolator (hopefully there is one) between the two could be malfunctioning, therefore not isolating any drain coming from the house battery. Batteries will try to equalize. I doubt your house/coach battery disconnect seperates this connection.
I wouldn't want to use a battery disconnect swich on the engine battery. Aside from losing radio presets/disabling remote unlock/etc, the car's computer (ECU) has to relearn your driving habits, etc every time you reconnect which in turn will give you poor performance/mpg/etc for a bit.
Simplified guide for tracking down parasitic drain.Anyway, with a new battery you may find your issue resolved.