My
Trillium is a very old one.
When I got it, the roof had 'dipped' in the front area (in front of the existing vent) and, in the back area there was a 'dip' of about an inch - plus a completely punctured area of about a square foot.
I shored the roof up on the inside. I sanded down all the multi-layers of
paint, then the gel coat (which was deteriorating). I then cut strips of 1/4" maple plywood, used epoxy to glue them to the shell, on the outside. I then 'glassed over them with epoxy resin,
light weight mat and cloth.
I did this on the outside because the ensolite had not been damaged on the interior and I wanted to keep it.
After I did this, removed the interior shoring, the roof has stayed the correct shape.