You don't heat a tent, you just sleep in it. Here's what I used to do (motorcycling around the place, sometimes in the snow).
Put a thin space blanket UNDER the air mattress. Put a woolen blanket OVER the air mattress, then your sleeping bag(s) and then a decently backed thermal blanket (it's like silver on one side and made of rip-stop). Don't wear a balaklava, the moisture from your nose will freeze the balaklava to your moustache (personal experience). Beanies are almost vital, as are woolen socks and open-fingered woolen gloves.
If you're feeling really dainty, you can get "pocket warmers" (Rays Outdoors, BCF etc will sell these) and put those in the bed. I did see a 12V heated jacket just earlier today but I can't remember where it was.
If you're hiking, snow-rated boots are great (don't wear them in autumn/spring, it's like you're walking around in a bucket of water - personal experience again).
As for heating a tent: don't. Just use the tent to sleep in. Because of the small air volume, you're compromising your health using any fuel-based heater, and electric heaters are usually crap. I have a 12V ceramic heater and it's garbage. 150W (over 10A) and it couldn't heat enough to be noticed with your hand right in front of it. And yes, I did turn it on.