For the
windows and skylight you'll want to remove, clean, and reinstall with fresh butyl tape. Resist the temptation to try to stop the
leaks with caulk, and, whatever you do, do NOT use silicone-based caulk. It leaves a residue that is very hard to remove and prevents other sealants from adhering.
The sag may be related to interior modifications by a previous owner. Most single-wall
fiberglass shells depend on interior floor-to-ceiling support designed into the original layout. My
Scamp, for example, has a full-height closet next to the door (a weak point) and a brace between the galley base and upper cabinets on the street side. If some of the original interior is missing, and especially if it spent some time in a location subject to snow loads, that could account for the sag.
You can restore the original height of the shell by gently and gradually lifting from the inside, but to keep it there you'll need to provide some support as you re-do the interior. As a starting point, you might want to search the photo archives for pictures of other Play-Pacs to get an idea of how they were originally built. The "Manufacturers" tab at the top has a link to Play-Pac, which should bring up some helpful information and links.
Some people who want a more open layout without floor-to-ceiling cabinets
fiberglass in ribs in the sidewalls and/or across the ceiling to provide alternate support.