Tent heater
 

I'm not sure I'd want to have a heater that I plug into my 12V power supply. Running a heater all night off your vehicle battery and then finding out you don't have enough juice to start your vehicle in the middle of nowhere is not my idea of fun.
Also your concern about a propane heater is very real. Campers die from heaters improperly vented every year. But there are a couple that are OK. This is the one I would use if I were going to tent camp. https://www.walmart.com/reviews/product/5712813
But I would also make sure I had my window flaps open a bit to let fresh air inside.

The problem with that MR Heater is that it's designed to sit on a solid surface and his tent is up in the back of his truck such that he isn't going to have enough room to safely have that right beside him. Also, that heater even on it's lowest setting would run him out of that small space in less than 5 minutes. IMHO his only choice is to get a small electric heater that he can hang above himself so he won't be turning or rolling over into it during the night. If I went that route, I'd plug it into a small generator that can be put out a little distance from the truck so it can't even be heard.

I've been using a Mr. Buddy heater in my travel trailer for years, and I'm not dead yet. Hooked to a 20 lb tank it will run nonstop for 3 days (All that proves is that I'm not dead...make your own decision). If I were doing a tent of that type, I'd get the Little Buddy unless you're going out in teens or colder. then I'd go with Mr. Buddy. Use your head & crack a window, and make some type of stand for it to avoid a fire. The generator/electric setup would work fine but I'd rather not mess with gasoline for any reason while big-game hunting. Good Luck and hunt safe.

Not advocating electric. But you could do a 2nd battery, and a diode. So start is not an issue.

As for generator, you would have to have such far away not to hear it, that voltage drop would be an issue, I suspect.

It's a small space and if you are just sleeping there I'd suggest a good quality sleeping bag to stay warm in. Much safer.

Mr. Heaters have Low oxygen sensors and shut off if tipped even a little. So, if you have adequate ventilation and maybe a battery powered CO monitor I think you would be safe enough. An electric space heater with a generator is also a good idea. Honda's are pretty quiet.

He hasn't been here since the day after he started the thread , in July. Either got his info, or didn't like what he read. Hard saying.