I used a small elecric heater at the Dead of Winter campout (Cloudland Canyon State Park, mid Feb.) two or three years ago. It kept the temp. in the bus at around 60 degrees even though it was about 20 outside. It probably would have kept the bus toasty if it had decent door seals and a few less rust holes in the front floor. But, I was hooked to 110 at the campsite. An electric heater is probably the best choice as long as you've got an electrical hookup.
I've run my key machine all day long using an inverter hooked to the battery and still had enough jucie to start Homer the Superbus (my world famous Type 1 powered 73 bus). I don't have an auxillary battery. BUT, a heater pulls a
whole lot more juice than a small key machine motor. I don't think a battery would last too long running a heater without the engine running.
Kyle and Betty use a Mr. Heater in their Vanagonagon (if someone thinks your door idea is impossible, they should see the Vanagonagon! . . . see picture below). If it'll heat the Vanagonagon, it should turn a "normal" bus into an oven! I think they bought their Mr. Heater at Wal-Mart for under $100 . . . I couldn't find one on the Wally-World web site though.
