I had the same problem with the SS Surfside
if you look in the last picture you can see I fiberglassed a 3/4" x 2" piece of fir onto the cieling, I used a 1x2" piece to hold it up just slightly "up" to induce a slight curve in the roof.
The skylight actually strengthens the roof as it has a nice aluminum fram and I cut out a large part of the sagged roof.
It is a huge improvement. I am going to replace all the louvered windows
as they are in poor condition. I plan to use tinted plexiglass for privacy as the skylight gives lots of light
The Daylighter skylight is the same one that they are putting in the new FRP Bigfoot
I found a set of mold for a FRP trailer near Vancouver and a boatbuilder friend of mine and myself are thinking of producing kits for do it yourselfers ?