We used to sleep in our XJ. If you slid the seats forward, there was an extra 14" of room, save the 7" or so in the center where the console was. I added a roof rack for the sole purpose of being able to throw everything from the back up onto the rack so it was off the ground with a tarp secured over it. There was around 6' in the back this way, and enough room between the wheel wells for two adults to sleep reasonably comfortably.
I haven't found as much room in our FJ, nor so the seats fold flat, but the rooftop tent is easier to setup anyway.