Sleeping in a 4R is similar to sleeping in a tent. Besides your good sleeping bag you need a good pad system. You can lose a lot of heat by conduction to the cold ground or truck floor.
Winter campers use two pads, a closed cell foam pad on the bottom and an inflatable pad on top for comfort.
Searching the forum you will find lots of recommendations.
Try the REI site for ideas. Be aware that good gear costs big $ but you’ll get many years of service from it. I use an Exped mat over a closed cell rollup mat.
https://www.rei.com/search?q=Sleeping+pads&t=articles
Good advice in the posts above.
Plus, you should crack some windows for ventilation, and maybe run a battery operated fan. Otherwise you’ll have a foggy drippy interior by morning.