The Mr. Heater Buddy heaters purport to burn more cleanly than, say, a camp stove. I wouldn't trust my life to the built-in oxygen sensor, though.

There's not a lot of air volume in a truck cap, so it won't take much to break the chill. If you're only using it to warm the inside for a few minutes while you change, I don't see a big problem -- provided you have excellent cross-ventilation.

It does pay to be paranoid though. I would run it on its lowest setting. A battery CO detector, which is not expensive, would be a very good idea.

I would not sleep with it running, period. Too much can go wrong. It could be The Big Sleep. Or a fire as you roll around during the night.