My girlfriend, 85lb black lab, and I slept on a double-sized (I believe) air mattress in the back of the 4Runner. We had the back seats folded down, and slightly under-inflated the air mattress. If it would have been totally inflated, the rear door wouldn't have shut all the way. With it under-inflated, when I shut the rear door it smushed the entire air mattress so it all fit nicely when the door was shut, and laying on it, it still felt "full". Hope that makes sense.
We had some room on the sides of the bed (where the wheel-wells are), but just used this room to stuff other items that we had brought with us. Besides waking up in the middle of the night FREEZING cold, it was fine. It was in the 20's outside, though. I think the only thing that could have kept us warm would have been insulating the glass windows somehow. I touched the window at night at it was like ICE.
We had one 15 degree bag, one 25 degree bag, two blankets, and the black lab. And we were still cold!