I've owned an '88, '95, '99, 04' and now a 2010 suburban as well as a 96 Yukon. slept in all of them planned and unplanned nights.
As mentioned, if you have barn doors you can't open it from inside. The lift gate if you have power doors and a remote fob you can pop the glass portion. my '95 was a plain jane work special with no plastic trim in the cargo area, I removed a panel on the passenger barn door which gave me access to the guts of the lock mechanism and if i recall was able to open the door.
When middle seat are in and in position, there is a good 60 inches from tail gate to the back of the seats. there is a 4-6 inch height difference between the floor under the second row seats and the actual bed. with the middle row back rest down but the seat NOT folded forward, I have put a double air mattress in there without problem.
I would suggest sleeping with your head towards the front, so that you can crack a window and get air. Another bonus is getting a piece of window screen and covering your rear windows so you can get ventilation minus bugs. If you are removing the second row seats you can put this bug screen on the inside on a more permanent basis, but if you want to keep the seats in and fold them, you can simply put the screen on the vehicle using magnets strips.
I have limo tint from second row back, i had it installed at a shop and the laminate they use is heat reflective. we slept in the truck for 5 nights while in Moab in late May comfortably. I have also slept in there in december while in Lake Placid with outside temps below freezing, and kept the inside warm by starting the truck with the remote start every 2-3 hours. It is still pretty bright in there, so if you want to sleep in past sunrise, the window covering suggestion is important.
Nic
2010 surburban