NM448: "An electrical heater with a small generator would be a good option for a truckbed tent. Don't go overboard, get a Jerry can and a ~2,000watt generator (pretty inexpensive), then a relatively small radiant heater. A Lil' Buddy will burn you out of a 2-4 person tent in a matter of minutes, maybe even be hot enough to melt the roof if you're not careful. I stay warm enough with a Lil' Buddy with my blind fully open, which is a lot bigger volume than most tents I've ever used. I like the Lil'Buddy heaters because they're smaller and more portable, and I have never loved the idea of using electrical gear on a camping trip, but my wife and I do have a 10,000watt portable and a couple convection heaters, all of it works. Insulate yourself from the bed of your truck - there's a lot of metal beneath you, and a lot of cold air blowing beneath your bed which will suck the heat out of you in a truck bed tent."
An electric heater and small generator are exactly what I told him he should use in the other thread he started. I've got a Big Buddy and a Little Buddy that I've used in two different size trailers and they aren't really for tents unless its an awfully big tent or they will do just as you mentioned.