You make some good comments!
We also have curtains to keep the cab blocked off when camping in the cold. We also make sure to keep the dash vents closed so cold air can't enter the cab through them. We also have a curtain to block off the cabover area in the cold. We carry a complete set of window bubble-wrap for the extreme cold if needed, but have only needed the bubble-wrap once. We would have to use our generator some every day though, even if our propane wasn't low. This would be not only due to the propane furnace fan electrical drain, but because our grey and black tanks have electric pad heaters to keep them from freezing.
Some of your comments started me thinking. You say that only the propane furnace supplies the neccessary BTUs to keep the coach area warm so therefore the generator by itself powering electric heaters is not an option. This situation doesn't mean that you can only keep warm until the propane runs out using only the propane. I'll bet your propane furnace could be used more days when drycamping if you ran it at the same time as the generator and electric heaters and thus keeping the propance furnace's ON cycles much shorter. Using these two together in combination may extend your cold weather drycamping time way beyond what you could get by using only the furnace by itself with the generator/electric heaters not being used.
Just some brain-storming on my part.