Do not assume that there is much structure in the slide exterior wall or TT walls in general. On our slide, there is only framing around the perimeter of the outside wall of the slide.
In the absence of available diagrams, watch on an early morning for the dew to highlight the location of the structure and take some pictures for future reference.
Here are a few from our 23RB:
http://www.keystonerv.org/forums/album.php?albumid=665
BTW: I installed access doors on a truck camper we had some years ago. It is relatively straight forward. Take care to cut the hole accurately and take the time to seal the door's frame properly and you will be OK. I took the extra time and removed insulation so I could add a frame around the opening. I felt I would improve the opportunity for a good seal by adding the frame in the wall cavity.