After a few years of contending with a few different brands of big inflatable mattresses that get random leaks and end up deflating on you in the middle of the night, I went and bought cots like
this.
Only works in a huge (8 man or larger) tent, of course, but since we're talking car-camping, I think it's an option worth considering. They take up a lot of space but it's also easy to keep the tent organized since there's lots of under-cot storage space. We've got a Coleman 8-man Tenaya tent and it's really spacious with three cots, we could probably do four if we needed. They also function as seating really easily.