New digital thermostat - no heater or A/C

[placenet]

First of all this is a 2007 Hurricane with low miles and in great condition. Here's the issue:
Old thermostat stopped working. All fuses were fine, even the one in the old Coleman. Assumed it was a bad board in the thermo.
New digital thermo hooked up, blew the 15 amp fuse twice. Power was only 10.8 volts. Replaced with 20 amp (risky I know) and both heater and A/C fired up fine.
Went on a trip. All good. Back home hooked up shore power and once again neither worked. Fuses, propane, batteries, generator all fine. BUT power at the thermo is only showing between 10.5 and 11.4 volts. Disconnected shore power and still no response.
I'm no electrician but wondering if the culprit isn't the voltage drop not being enough to trigger the units? If so what should I check next? Or any other ideas?
Thanks for any help. I've learned so much through this forum.

[16ACE27]

What's the voltage to/from your Furnace? That's where the thermostat gets its power from.

[placenet]

That's the 10 to 11 volts I mentioned. No reading was above 11.5. Thanks for any input.

[placenet]

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!