I have experienced both sag and cracking. I suspect that I got lots of years out of the roof (as did the PO) because of the covering when not in use. That said, the cracking started on the side that held the bag awning for my Coleman Niagara. There has been some separation (delamination) of the ABS coating from the subroof, but the layer has remained contiguous. So I chose to focus on the cracking, rather than stripping the surface or trying to re-adhese the layer. There were areas that I injected glue under the surface. I chose not to go the ABS pellets and MEK route, primarily because the volume of material I needed was small. I found a product called "Bondaide Platsic Glue" that is a two part epoxy putty. It is important to find every crack and drill out the ends and widen the crack - I used a rotary tool to stop the tracking of the crack and to get space for the putty. one part is wrapped around the other and you slice off part and need it together like playdough. It hardens fast (very short working time), and sands well. I use truck bedliner (Hippo Tintable) and given the size of surface used the 2 kits in the package. I was very impressed with how well it went with rolling given short working time. And the sag is more pronounced due to the A/C despite the support bracket. But simply using shower curtain rods the screw type with metal inner rod (stronger than the plastic screw) allowed me to counter the sag. The roof becomes more plastic in the hot summer heat, so using the screw mechanism and some cedar shims, I have "corrected" some of the sag. The rods are place under the ends of the A/C support bracket to live from the counters. There is enough flex in the roof that the force on the counter tops has not been a problem. With respect to the bedliner coating - it was a bit of an experiment for me and worth the risk to preserve some value considering the cost of replacement for a roof or the trailer. The cost of materials was not expensive; it worked out in part because the damage had not progressed too far, and I spent time planning and preparing. If you decide to do this on your own, spend your time in preparation and research. Seek advice as there are different solutions that will work. I am so far very happy with the results.