I’ve been studying that for a while now. At a block party last summer, us neighbors shared our propane bills and found them to be comparable. We have similar model homes in a 25 home cul-de-sac. I also monitor the price of propane regularly.
One thing I am suspicious of is that the heat could be going right out the windows since all the floor vents are situated next to windows. The two space heaters keep the living spaces warmer with much less effort than the furnace.
I’ve checked for adequate and correct attic insulation, I caulked around all my windows, I even put up insulation on the windows last year to no effect, and I checked the whole house for drafts and leaks without finding any. I just think the whole system is somehow inefficient.
I know that seems counter-intuitive but that is normal. The idea is that windows chill the air and cold air falls so the heat vents are located below them to put rising warm air there cutting the amount of cold transferred from the window to the air.