Originally Posted by Flybob
I assume you are plugged in as you were running electric heat. You say you have less than 12V on the furnace power then that is your problem. The furnace and fridge have a safety feature that will not allow them to run if the 12V source is too low. Check your converter, the high current battery protection fuses and the battery. What is the voltage at the battery terminals. Is it possible there was a power surge/failure when you were away and a fuse blew.
As stated I have a terminal marked +7.5 VDC and it has 7.5 to 8.0 VDC on it. I had considered changing this old analog stat once and this is the voltage it had then and the heater worked right, with no problems. The AC also worked as it should.
My thermostat has six terminals. They are marked as follows from left to right. +7.5, cool, fur, hi, hi, fan, grn. Reading voltage from the terminal to the grn terminal (this is the negative battery terminal) I read +7.5 VDC, +7.5, 13, 13, 13, negative (ground). This is with it connected to shore power and the cool / heat switch set to off.