could kill you, the smoke emitted from a burning candle will kill you in your sleep as they produce carbon monoxide when burned (paraffin, so in order to avoid that you would need to keep a window cracked and any heat gained by the candle will be lost with the draft.
Colman and buddy both make heaters that work on propane that can be used indoor, just make sure you get the right ones with the cutoff sensor in them, since the cab is so small just get the smallest one you can find and set it up in a place that won't tip over or melt anything.
really though, quit being a p***y and just get a decent sleeping bag, heat the truck up before you go to sleep and keep the keys handy to fire it up right when you wake up.....you really shouldn't need any ancillary heat in the first place if you're camping with the appropriate gear, and really, you'll likely get more use out of a good quality cold weather bag than you would some propane heater or the like....i've used my -30f bag all the way into the 60's and been fine, at that point just open it up and use it as a blanket instead of a mummy sack.
