I tried to lay the small seat flat and it doesn't show the true slant in these photos, but it's definitely there. I got in and lay down with my feet elevated and head at the rear of the cargo space, and I fit. The problem is that my air mattress wouldn't begin to fit, even though I have a twin mattress.
So to be able to use the mattress I would need to let down the larger of the two back seats. Also, Ara pushed her back seats as far forward as they would go, leaving a good sized gap between the seat the the actual cargo area. She filled that with pillows to close the gap. I think rather than move the seats all the way up I would just use the floor space available for storing camping gear, beer, etc. I'm not sure how I will fit a big cooler in. I have a medium sized plastic cooler and a large Coleman steel belted cooler (replica of the oldies we used to use). I think for a really long trip I want my Coleman but the thing is too heavy for me to lift when full.
Another couple of negatives I have is that the hatch can't be opened from inside. That should be ok because I could exit out the rear side door. The other big drawback is that the hatch back window doesn't open, nor do any of the windows on the sides of the cargo space. I really don't like the idea of opening a window more than a tiny crack, especially when my head and torso are in the rear of the SUV. Not even sure if any fresh air would get to the back.
Then there is the problem of covering the windows. I could go buy some velcro and some cheap fabric. Ara and Hannah rigged up and hung towels on the side windows, and I guess I could do the same but I'd have to look at it more closely.
I don't think the Highlander makes the best vehicle for sleeping in, except in a pinch. The first picture is taken from between the front seats and looking back on the passenger side. The other two are taken from the outside back of the vehicle. I went back outside and took two more photos trying to show the slant of the seat and how it doesn't lie flat. Not the greatest shots, no doubt because I'm trying to take them when the car is parked in the driveway - no room to really get the various angles.
Comments and suggestions welcome, but please be nice.