The original set up on the Trailswest Campster has a full height floor to ceiling wall at the end of the kitchen and closet sides. That wall location is 3 feet away from the back wall. In the original is was made with 3/4" plywood. As to getting the height and the contour shape of the partition. The inside of the rear wall of the trailer can be used to create a new partition pattern from cardboard as the shape stays the same. You will of course need to also make a pattern of the contour of the wheel well shape to cut that area out of your partition as part of your pattern work.
If you are having a lot of sagging issues where the pop up top is located part of that problem might be the outside wood frame for the pop up that sits against the roof. That wooden frame needs to be really strong as it is doing a lot of the work of keeping the roof from sagging along the longer side of that opening. I had rot in that lower popup roof frame so I built a new one. Because that frame sits right on the surface of the original wood it is long past the end of its natural life span. The height of that divider wall is easy, it matches the rear wall in contour at the roof line. So just use the back wall to make a pattern of the shape using cardboard and transfer that pattern to the new partition. You will need to make a second pattern to put at the lower outside edge of the new partition wall for clearing the wheel well.
I made my new frame from 1 x 2 Ipe wood. That is a very strong and dense wood that has a lot of natural rot and bug resisting, oily resin in it. That wood is used for outdoor decking on high end houses. You will just leave it natural, it does not need to be painted and since it has so much natural oil in it paint
won't stick to it anyway. You will not need to use adhesives, it is just screwed together at the corners but you will also be screwing into it from the interior so it won't be going anywhere. You can buy Ipe wood at lumberyards but you won't find it at places such as Home Depot so go to a "real" old fashioned lumber yard. Building the new frame only require very basic skills as you just have to cut the pieces to length and then screw it together. The long sides overlap the ends of the shorter sides. You must use stainless steel screws with this wood. That includes using stainless screws for attaching the hold downs and trims that are going to be going into the wood from the interior.
Yes the roof does slope. Just go to the front wall of the trailer and look at it. It sloping from the center as the high point when looking at that front wall straight on. But if you build a new lower frame around the pop up opening you don't need to worry about including a slope on it. It will all work out just fine to keep those two short end boards their natural straight edged shape as it is a very gradual slope.