Last time I went camping I got lazy and slept in the back of the XJ. At first I tried just sleeping on the cargo floor with the bottom of the rear bench removed. Probably the most uncomfortable attempt at sleep I've ever made. Out of desperation, I grabbed the Coleman full size (double) air matress that I usually use in the tent and threw it in the back of the Jeep. I found that if you partially inflate it, you can make it work. Getting onto it is a little tricky (it bottoms out wherever you are crawling) but once you lay down, the air pressure balances out and it feels like you're sleeping on a water bed. Tossing and turning will make it bottom out as well. Not ideal, but it worked in a pinch.
BTW, I'm 5'8" and I had to sleep diagonally with my feet hanging over the end of the rear seat back to fit. There was just barely enough room for me and my dog back there.