The boondocking post is an excellent read. What I've wondered about adding, provided there is a way to properly install it is a vented Platinum Cat heater. This should give the nice heat of a propane heater, while exhausting the combustion gases outside, and wouldn't be draining the battery like a conventional furnace does. It does use 5 amps at 12 volts for five minutes, then drops to 0.5 amp-hours once the exhaust fan is running, mainly because all the fan is doing is pushing exhaust gases out, not forcing air through vents and a heat exchanger.