I spotted an old Woods down-polyfil blend bag in the local second hand shop yesterday. That sucker is flatter than a pancake. (It's more like a crepe.) At first glance, it looked more like a tarp than a sleeping bag. The outer shell is in fantastic shape, and looks like it provides a good vapour barrier for the fill. For $10, I'll try to restore the loft and, if that fails, it should make a good ground sheet. Either way, it's too heavy for anything other than car or backyard camping so it's destined for a home in the back of my Jeep.

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?

Army wool blankets?

I have one in each vehicle year around,

I have one in each vehicle year around, supplemented with knit hats, wool socks, and old pac boots.