PT - I have to give you a huge amount of credit for doing this. When temps get down (and not even close to -40ยบ) I'm at home in the S&B with the wood stove burning quietly away.
One poster put up a link to a propane stove that uses no power but needs to be vented, but I'm not sure if it's straight up vent (ie roof penetration).
The other option, if you can find the right spot for it to work, is a vented heater similar to this:
http://www.northerntool.com/shop/tools/product_200362089_200362089
I'd be looking for something like this in your situation. I too won't go the route of a unvented heater. And I want something that doesn't use any electrical power. Now if it until like this would keep you warm, your batteries can run your lighting, bed heater, etc. Drive time, solar and the occasional 15a shoreline to keep charged no seems so much more do-able. Years ago we had a venter propane heater similar to this built into our pop-up. It was only 7000 or so btu but really made a lot of warmth with no power draw. Miss that little heater.
FWIW - I'd also consider a small, VERY inexpensive genny. Something like what Harbor Freight carries for ~$100 USD. Yes, its two-stroke, not super quiet. But I look at it as insurance.