I have to admit, I really don't have a solution for this. At best, I have 1-3 light blankets and maybe a space blanket or two (the junky kind) in any of my vehicles at a time. If my family & I were ever trapped in my vehicle we'd primarily be relying on our clothing to keep us warm.

The big challenges I see here are: 1) cost, and 2) space. I have 4 people in my family therefore to be properly prepared I'd need 4 sleeping bags per vehicle (so 8 total). Even lower-cost synthetics might set me back $800 and then they'd end up take up most of my day-to-day storage space.

For these reasons, I don't see sleeping bags as a realistic vehicle prep for me. Maybe something like the SOL emergency bivy might work for this?