I guess it would depend on exactly how flat you can get the "floor", as to whether or not it would actually be comfortable to sleep in.
As I see it, you'd have to remove all of the headrests, move the front seats right forward, then fold the back seats as flat as possible. Then you'd need to push the front seats as far back as they'd go, before folding them down too... which I reckon would leave it fairly flat?
Chuck in an air bed (would a regular double fit with a bit of squashing?) and I reckon you'd be fairly comfy! As you originally said, it has to be better than sleeping in a tent whichever way you look at it.