Hoping this helps others with a similar issue. Turns out this was not due to a voltage drop. The 12V power comes from the house batteries, through a 15 amp fuse on the electrical panel under the bed, directly to the heater, and then to the theromostat and finally to the A/C and back to the heater.
Fixed: remove the exterior heater panel and look for a tiny switch. Its not obvious and can be red or black. Its a simple off/on mounted laterally and not easy to spot. Make sure that it's not moved to the off position, or wiggle it if its on but not connecting. Voila! power restored to the system. Why there is an on/off switch there is beyond me but there you go. Of course it's not mentioned in any documentation!
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