This question comes up a lot. If you have 110V power, an electric heater can't be beat.
But both a catalytic heater and a furnace have there drawbacks.
A catalytic can kill ya, (very bad) and a furnace without 110V will eat a battery in a hurry. Forget what the charts say, a group 27 battery out in 20 degree weather won't last an entire weekend.
I use a Catalytic about 2-3 hours before I go to bed, shut it off and then get under a lot of blankets.
In the morning my stove slides into the cabin and I make coffee inside. That really heats the place up.
I agree this could be made with some dryer vent tube, a computer fan, a Coleman single burner stove (there 10,000 btu also) and some kind of heat exchanger.
There are lots of other "Low-Tech" ideas as well. A hot rock, Hot water bottle (made out of plastic juice bottles), a cast iron frying pan works great as a bed warmer, even a dog is good.
But my all time favorite is another body.
Happy Trails
Len