There's a really interesting thread on here on CO that goes into both generators and charcoal (as well as chemical). An issue that was mentioned was the person opened the car door, got in and immediately passed out, didn't even have time to close the door. Some have suggested wearing a gas mask to try to avoid that problem. Other issues are parts of the car (or, in the case of a tent) melting, and the car becoming too hot so the person bails. Finally, someone recently mentioned something about charcoal now being made from coconut, which apparently does not emit as much CO. Sorry to be a downer, I've just read that entire thread several times. There's really good information in it.
Another thing I was thinking is it would kind of require pretty private property to pull it off. I don't have property and when I looked on google maps, I couldn't find any private area anywhere near my location.