Hilldweller
SE Expedition Society
Big truckstops have 12v versions of them.Can you plug something in? I use a little $15 electric heater from Walmart in the camper in the winter. It's 110 though, so if you can't plug in, that's not a solution. I use my coleman lantern to warm the tent when it's really cold, but you have to keep it vented. Really, any easy portable solution is going to require some ventilation... And anything 12v is going to pull a LOT of power from your batterie(s), and will probably run them dead in an hour or so if the engine isn't running.
Good luck!
They're power-hungry though.