Guy, we bought a Surfside
that was, sadly, in much worse shape than we anticipated, and are in the process of doing a complete tear-down and re-build, including a new floor. Lots -- lots
-- of work.
In the process we removed everything from the inside, right down to the fiberglass, and will soon be building new cabinets and counters, which means we had to make new supports for our trailer sides and roof.
I built new rib/formers that support the sides of the trailer. There are five of them: Two full-length, extra-deep (7-1/2") ribs, one on either side of the door, one full-length rib on the street side to one side of (what will eventually be) our front dinette, and two short-ribs that sit over (but do not come into contact with) the wheel well.
These are positioned so they'll hold the side contours of our trailer in and support the roof, but also provide connection points for our front door hinges (the "wings" built into that first rib on the left), screen door hardware (between the first and second rib, a (curb side) refrigerator
cabinet, overhead cabinets, and (street side) kitchen counter. The holes I've cut in them make them lighter, but also prevent the boards from warping and provide spots through which fiberglass cloth loops can be run to tie the rib to the wall.
The picture was taken after they were glued into place (using either clamps, as you see on that first rib, or shell-penetrating screws that were later removed and Bondo-ed over), and before using fiberglass mat to tie the ribs anf hinge wings to the wall. Once the mat was in, I cut the excess mat from the holes and threaded additional 1" strips of fiberglass cloth through them to the shell to further strengthen the connection between shell and rib. (Massive over-engineering, that last step.)
I also added some fiberglass stays to strengthen the roof.
These are made from lightweight 1" PVC irrigation pipe that has been cut in half and fiberglassed to the roof. They're very lightweight and don't look like they add much, but the look is deceiving. They add substantial strength and made the roof much more firm and resistant to warping.
That last stay is in the process of being installed, and is being held in-place by a telescoping tent pole and broom handle while strips of fiberglass on either side of the temporary supports hold it in-place. You start by supporting the PVC half and lifting the roofline up as much as you can with the telescoping supports then fiberglassing the center portion into place. Once that's set solid, do the ends (as shown), then after that has set solid, cover the whole piece of pipe with a 6" strip of mat centered on the PVC pipe and ticked tight to pipe and shell.