I have slept twice on my fold-out sleeping platform. Even laying on our sides, the slight angle of the upper body has not been noticable. I liked the idea of being automatically protected from the weather and whatever else, and not having to deal with setting up tents.
So far we have only done it with a couple thick comforters as cushion, but I am toying with the idea of a thin full-size air mattress.
My wife has discussed a RTT, but it was more in passing.
While not particularly safe, on one long overnight trip we took turns sleeping on the platform in the back while the other drive, and that was basically heaven relative to sleeping in a reclined seat.