There are some fundamental problems with using 12V to heat anything - you need alot of amps. There is the Koolatron 12 Volt Car Heater, but it seems to be designed to clear the fogging of rear windows and not necessarily heating whole tents. There is also a small 12V ceramic heater (see link below) , but it needs a vehicle alternator to produce 40amps to function as a heater:
http://www.safehomeproducts.com/shp2...2/377/377.aspx
You might be able to run this ceramic heater off of a deep cycle gel battery but at 40 amp/hrs its not going to last long (maybe 2.5 hrs out of say a 150amp battery, as you don't want to discharge it too deeply).
Tents will not retain heat well (no R value to speak of), so the best heat generator in a tent is your own body (if enclosed in a good sleeping bag).