I keep a sleeping bag in the vehicle for these occasions, as well as emergency rations, a small stove and the makings for hot drinks. A bag appropriate for conditions is all that is required.

I am cautious about CO - ventilation is all you need, but how much can be problematical. I had one experience where I thought ventilation was adequate - Surprise! - I was wrong (not vehicle related, by the way). CO can be very insidious.