This video applies only to base camping canoeists in cold weather, but has some good info for the less experienced about heat flow, condensation, bag and mat insulation ratings, and safety.
I have been sleeping in tents since the 1950s and have spent a lot of time sleeping in the open. In the last 20 years I have mostly slept in the back of pickup with a canopy on it except for river trips in a tent. It helps to bring plenty of pads to sleep on and insulate your body from the cold steel of the truck bed.
My wife and I slept under the cap of my pickup on our way to Mad River Glen, Vt. When we got up I was able to start the truck through the sliding window and we waited for the cab to warm up before getting out. The radio station in Rutland said it was minus 16. That I believe was my wife's last winter camping experience.One time I slept in my car near Jay Peak, VT in temps that reached -13F. I cracked the windows and used a -30 degree sleeping bag and slept remarkably well.