Oh, can I relate to this. Although not from a living standpoint, more a camping/not camping type of situation.

In my two most recent cars, which I have attempted this with, the first was most definitely better. It had a small lip between the folded down seat and the trunk space. Easily overcome with a layer of foam in the trunk. The biggest challenge was the size of the opening only just allowed me to turn on my side.

The second car I tried this in, which is still my current car, is rather impractical. The seats don't fold flat and there is even a angled hand foam piece from the trunk floor to the angle of the seat, which is about 30 degrees. Not comfortable, and the opening is not wide enough to sleep on your side, it is on your back and that is it.

It was fun reading and I would have to agree that a van could be a much better option. :)