OK, my weekend endeavors were fruitless. After tracing the lines and disassembling the second battery housing (PIYA!!!) I at least know that the second battery is now completely dead. But I do not know how it happened or what drained it. (it was only a year old - from the dealership)

I did discover that what ever it was, when I put our battery tester / charger on the unit to figure it out... There was a loud pop and sizzle followed by a lovely plumb of smoke from my, now DEAD, battery charger (RIP).

Completely baffled and no closer to figuring it out, I gave up until I can replace the battery and try again.

Solenoid? Relay? I dunno. Does anyone know how to test those buggers?

Sigh.............