I might disagree, I think. When stowed, the slides on our rig are flush with the rest of the body. When deployed, there's a definite shift in slope to put the outside wall at a slight angle to the main body. The top of the slide goes out about a half inch further than the bottom.
When we were shopping, most of the rigs we looked at were the same. The explanation I got, from the previous owner of the rig we have, is that this slight tip angle was a means of making sure any rainwater on the extended slide ran off rather than into the rig.
When I put a level on the bed, it seems to be close to level when the slide is out. I'll double check on the dinette slide when I've finished my rework on the slide-out cabinets.
Frank and Eileen
No longer RVers or FR owners