We've been planning on car camping for portions of our road trip in North America. Fortuitously, we also need to discard the fold-away bed portion of a sofa which I modified, replacing the fold-away section with simple (light) plywood. The bed portion has been in the way in the garage for months, and today I decided to try the mattress in the back of the Subaru--it's a small full-size mattress, and it's a perfect fit! We were worried earlier that the bump where the back seat bottom folds up would restrict our headroom, but it actually works very nicely as a pillow bolster.
Now, this definitely would not work for a 6' tall, back- or front-sleeper, but we're both side-sleepers, and it was quite comfortable with both of us in there. Ruby even hopped in and settled down. It did get hot pretty fast, so we will need magnetic bug screens, and some way to regulate the window opening without clambering into the front seat to turn the ignition into the ON postition. Definitely an instance where crank windows would be convenient.