The vehicle battery in my '84 Westy is charging ok, but within a few hours
of sitting, goes down to about 12.35 volts.

Alternator is new as of July, and at idle it's charging over 14 volts at the
cigarette lighter with no accessories on. The aux battery stays consistently
at 12.8, reading it after 12 hours of sitting. That tells me the charging
system is doing its job and that the aux battery is in good condition.

Measuring resistance  from the vehicle battery positive wires to ground
(disconnected from battery)  gives 0 ohms for the charging wire, something
like 12 milliohms for the "load" wire, which drops to 0 when the fridge
LED's are turned off. In other words, no abnormal load I can see.

Still, no matter how much I charge it, the fridge LED is always on yellow
(below 12.5 volts IIRC) within an hour of turning the car off, with no
accessories being used--even the LED's.

Battery was new in July.

What gives?