Pretty much agree with everyone. A low amp 12v heater is a hen's tooth by definition.
Here's what you need:
http://www.coleman.com/coleman/colemanc ... sCjjz1FudA
Not trying to be pithy, but our Coleman big game sleeping bags are one of our best, if not the best purchases we've equipped the SMB with. We paid $50-ish on the costco web site, probably not going to see that again but these are fantastic. I'm 6'5" and I have tons of room plus they can be zipped together. We have 2 for below and 2 for the penthouse.