My understanding with the co method is it's pretty peaceful. Depending on where you do it, and having all cracks and openings sealed, it can be successful. I've read about people in China who ctb using charcoal in their cars.
Some years ago, I read an article in the paper of an accidental CO death of a girl while camping. Apparently she was cold, and her dad moved the charcoal grill closer to the opening of the tent to keep it warm. The fumes crept into the tent and eventually she died in her sleep from inhaling the CO.