Sleeping bag or go heavy on blankets for truck bed camping?

I plan on doing a lot of car camping this fall and winter around the western and desert states. I'm staying away from snow as much as possible, I don't do snow. I have a shell (topper) and I built a sleeping platform in the back with a mattress. The whole back is uninsulated and I don't know how cold it gets back there since I only gotten the shell a few months ago. Its an oven in the summer.

Do I get a nice sleeping bag or go with some regular twin size blankets? I also have a Buddy Heater on hand.

Sleeping bag or go heavy on blankets for truck bed camping?