When I travel I sleep behind the wheel anyway, even in super cold and snow, so I want to be comfortable, which of course means that mere survival isn’t even a question.
I have an extended cab pickup truck, so when I sleep in my truck on trips I move over to the passenger seat and push the seat back as far as it goes, and recline it as low as it goes.
Wool clothes and wool blankets and I’m pretty toasty.