Well, I am 6'4" and 210 lbs, I slept in the back of my SX/4 on a road trip, It was cramped but worked in a pinch. I did have my feet between the seats, both seats were inclined forward and the transmission in 1st gear with the emergency brake on. never felt concerned about kicking it out of gear. I did end up sleeping with me feet against the driver side door and seat. I wouldn't ever plan to camp like that, I did it just because I was falling asleep behind the wheel and just had to stop to get a couple of hours of rest.
I did camp out in the 84 wagon I used to have and it was pretty good. just put a 2" foam pad down and left the rear hatch open for air.
I camp in my 1964 slide in Travel Queen Camper now. It is really big for a camper, I load it into Big Red, my 1992 Dodge D350, 12 Valve Cummins, Dually, and pull the SX/4 or my Scout behind for running around or trail fun when I get where I am going.