On my '95 EVC the relay that allows charging the house battery when the
engine is running simply connects the vehicle and the house battery in
parallel. The cable to the relay just comes from the positive terminal on
the battery. The relay is activated by the ignition. And you're right--a
discharged house battery would discharge the vehicle battery if the ignition
switch was left in the on position without the vehicle running.
Marve
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'95 EVC
'82 diesel Vanagon