Yeah I agree with the others, down in the lower 48, a catalytic propane heater is all you need.
Normally a Coleman Sportcat is all I use, but if it's really cold I'll run a Mr. Heater Little Buddy on a 2gal tank w/ hose
I frequently hunt out of a my Dodge and because I also have a large camper for it, I chose to go a different route. I don't have the shop space to be pulling a shell off/on when I want to load the camper. So what I did was build a modular shell solution. I wanted racks for the truck for obvious reasons and decided to take it to the next level by adding a canvas top over elec conduit roof bars. My brother calls it the "Conastoga".
Here's some pic's of it on my old Dodge.
This modular design allows me to take it on/off at will with little effort & about 30min. Stores well, is height adjustable, and the corner joints are held in place by door hinges that I drift the pins out of to disassemble. I've since added a hinged back to it as well. I use the hell out of it. I've also been wanting to build a hanging bed platform but haven't gotten around to that. Right now I just roll out my cowboy bedroll and throw down on the floor. Speaking of my bedroll it consists of two cheap warm flannel Coleman one inside of another, on top of a 6" Coleman mattress, w/ my
DIY Gore-Tex Cowboy Bedroll Cover over the top. I sleep like a baby regardless of the temp w/ or w/o the propane running.
Anyway, just thought I'd share a different solution to your needs.
I get a lot of looks and comments on it and guys are always asking to look inside to see how it was built.
Hunt'nFish