2001 Rialta battery won't hold charge, can't be jump started, can't be charged

[Gumppy]

2001 Rialta purchased December 2009. Believe all equipment is original, prior owner was second-owner and hadn't changed anything. Through winter started engine monthly, no problems. Spring getting ready for trips note that battery was dead (no digital gauges, no radio, turn key, no click, no start. Jump started the battery, reset clocks and radio, drove it about 20 miles to charge the battery. Rialta did OK (monthly starting) for a couple months. Again get into cabin, no digital gauges, no radio, no click or start. Tried jump start, no joy. Tried battery charger no joy. Tried Aux Start button to bring deep-well battery online. No joy, no click, nothing. Volkswagen dealership won't look at it. Local garage who does RV's and he's stumpted. Tried a new battery with a full charge. Next morning the charge had run down. Really want to take this beast out in a couple weeks. Any ideas?

[Gumppy]

Replaced all the batteries, placed a blade switch to isolate the coach batteries when beast sits idle for a while. Still a power leak somewhere that will discharge the chassis battery so must start the motor to charge it up every month. Still looking for the electrical thief somewhere in wireing. Anyone have problems with the wiring loom for the towing hitch in back. For some reason, a previous owner cut the wiring.