Here’s another thought. From where does your propane furnace get its combustion air? Fire eats air, which will be made up through crevices in the construction from the outdoors, which is cold. Electric heat eats no air. Ideally the furnace would be located in a room sealed off from the rest of your living area, having an outdoor vent to get combustion air with. Then the cold air isn’t mixing with your living space air.
I don’t know much about furnaces. The furnace itself is actually in the cold concrete basement. The basement temp probably remains at 45 degrees in the winter. Now, there is a large vent in the living room which I’m guessing is some type of feeder since no air comes out of it when the furnace is on and I think I feel air moving into it.