I have to agree with what was stated above, that 12V heaters are battery killers, and sleeping inside a truck in a cold environment is a bad idea all around. How do I know? I slept in the back of a pickup truck for two years when I lived up and down the coast being a surf bum, with trips to Baja and the Mojave Desert. For six more years, I camped in the back of the truck. Then, I camped in the back of my Xterra for a year and a half on several winter desert trips. I now sleep in the back of the Cruiser, but only on balmy nights when cold is not a factor and I don't want to deal with bringing a tent.
In cold conditions, the metal cocoon that is your truck will sap the heat right out of your sleeping mat, your bag, and eventually, you. I learned this the hard way. Many a night, I woke up at 03:00, unable to feel my feet, while my buddies in their tents slept like cozy babies. I'd get up and crawl to the fire or a butane heater to try and heat up my toes. A couple of times, I became very sick from hypothermia, and my compromised immune system that resulted from night after night of fitful, freezing sleep resulted in trips ending early. It should have occurred to me earlier that sleeping in the truck was the culprit, but as a Middle Eastern guy who is far more comfy in 110ºF hell than in cold weather, I always assumed it was because I wasn't "made" for cold weather.
I experimented with heaters, but none worked to my needs. The 12V heater was wuss, barely heated at all, then died around 02:00, which left me cold for the rest of the night. Then my buddy lent me his "heater buddy", which ran on propane and had some catalytic doo-hickey that meant it could be used in a tent or vehicle. That damn thing was hotter than fire, and I was afraid of the CO2 or burning my bag or self up, so that was a no go. Then I tried waiting until I got cold and woke up, at which time I would stuff my socks with those heater coal packets that you see in survival kits. That worked okay, bu my thighs and ass were still freezing, and I didn't have enough coal packets.
My friends couldn't figure it out, and kept calling me a Nancy Boy for always neing cold, until one night I slept in a tent, because I knew the temps would get down around 0º, the coldest temps I'd ever camped in, and I figured I was better off next to a human being who I could turn to and say "I'm dying of hypothermia. Please help." Guess what? I slept better than if I was at home in my own bed! I was sold on the tent thing, no matter how much trouble it was to carry and set up.
