I did a little websearching and found some tent heaters that could probably do fine in a truck camper as well. They're mostly propane-powered but there's a kerosene one on the list with a run time of 8 to 12 hours that's beefy enough to do a whole cabin. Some of the ones listed include oxygen sensors to automatically shut off if they detect poor ventilation, probably a good idea if you're leaving it run overnight.

I suspect that the best approach if you're trying to "improvise" and are worried about poor ventilation would be to not bother keeping the whole space heated overnight and just invest in a really good sleeping bag or some really good blankets. You won't care if the rest of the camper is cold while you're unconscious, only keeping yourself warm matters and with enough insulation your own body heat will suffice for that. I keep a rolled up set of wool blankets in my car just in case I end up skidding off the road in a snowstorm and need to spend a while waiting for help to come, for example. Wool blankets can be quite surprisingly good at keeping you toasty in a cold environment and can be used when you're just sitting around during a frosty day, too.