Bill, I am not sure, but looking at the picture that you have in your post, I would expect that the panel is held on in several key points, and it is probably held on underneath the trim, such as the wood rail, and around the window "surrounds", that is those trim rings around the picture windows. I have a similar interior in my van, and the trim rings around the window are held in with long sheetmetal screws that tap into holes in the ribs in the wall of the van. It might be helpful for you to look at some of the other member's pictures of untrimmed cargos, so that you can get an idea of where the ribs are in the side of the van that support and form the framework for the outer sheetmetal, if you know what I mean. Most likely, the reason you are seeing the sag in the fabric that the picture looking into your slider door opening shows, is due to the fact that the adhesive that was used to attach it to the backing panel, (either wood or more probably fiberboard), has dried out over the years, much like a headliner does at times. Don't be concerned about that too much, as you can repair it rather easily, even if it is rather a tedious operation. If you peel back the weatherstrip around the opening, (don't worry, it comes right off and goes back on just as easily, it is only an interference fit, no glue or fasteners), and you will be able to see what the trim panel is actually made of and how thick it is. This should help you to figure out where the attachments are to the ribs, etc. I can tell you that from my experience designing and assembling vehicles over the years (30 +) that usually there are fewer fasteners than you might expect, as a multitude of fasteners adds significantly to assembly time and cost! Usually the fasteners in these conversion vans are sheetmetal screws about 2" long, some longer. Some of them might well be self-drillers as well. They might be hiding under decorative buttons that simply pry off with a small screwdriver. Look carefully, you will find them. Keep us posted!
Edit- I looked at the pictures again, and that little armrest thingiee with the ashtray in it is also gonna have to come out to get the trim panel off, you'll probably find some sort of attachment screw, likely to be about 3/8 " diameter with a real coarse thread on it under there. I'd look carefully at that first. I think you'll surprise yourself if you look carefully at it.
Mark
![Shame on you [-X](./images/smilies/eusa_naughty.gif)
1991 Hartland EXT with Z motor. Only 190K miles on mostly original parts.