I had the same problem with my HW256. There was a noticeable sag of the roof edge from front to back. It was just something I learned to live with. I doubt there is a fix.
I do not understand how this problem can develop as pop ups tend to spend most of their time closed up and the roof edge is resting on the side wall. I get it if the pop up was up all season and the weight to the A/C unit was pushing down. Then the 12'+ span between the corners could enable sag.
I hope my 16' box roof does not develop this as well but I suspect it will.
I am planning on adding roof braces crosswise to support the roof better to stop the center from sagging. I wonder if I could run some plate metal along the inside as well to give support to the wood. My understanding is that the sidewalls are basically just plywood.