The guy made some good points about air quality/safety issues.

My buddy a few years back went to the UP of MI to deer hunt. He set-up camp on a 2-track and had a tarp/fire-pit set-up and was going to sleep in his pick-up cap. He had a cot and a good goose down bag and one of those Little Buddy propane heaters. They have a low-oxygen shut-off system and are advertised as safe for tents/ enclosed areas.

Problem was that between the unit shutting off and the level where the air gets bad enough to make you feel ill leaves a lot of room for error.

It got down in minus degrees, and when the wind picked up, he felt a draft, so he closed the caps window completely, figuring there was enough leaks, that air quality wouldn't be a problem.

Turned out it was. He woke in the morning with a splitting headache, sick to his stomach and missed a day of hunting.

That night he re-arranged his sleeping position so any draft from his vent window wouldn't blow directly on him like it did the night he closed the cap completely.

I don't know if the air quality would of gotten to the point his life would be in jeopardy before the unit shut-off, but mechanical things fail all the time. I think I'd error on the side of caution and make damn sure the enclosure was vented properly.