I gave my dad a Coleman cat. heater years ago. Don't remember what model, and both he and the heater are long gone, so I can't ask. It had a heating panel about five inches by ten if I rember correctly. Dad said that it heated the inside of his camper shell, which was on a full sized P/U (short bed), just fine. I assume that it's output was in the 800btu range, so the one you are interested in should help keep the inside of a vehicle a little bit warmer, altho probably not as well as his fiberglas shell, since you will have all that glass around.

I'd keep a window cracked on the lee side, just to be safe...