I do not buy pre-made casements in the way you would for a conventional trailers or homes...After reading my post again I can see how you would think I did.
I do a build up process to create a casement out of the hull...I cut my f.r.p. "fiberglass reinforced panel" strips 2' wide, the long measurement at the top of each window extends out from the window cut out by an extra two inches on each side like ears, this is so the 2' sides I also will lay up butt up exactly against it, I rough up with my sander the textured side of the f.r.p. strips, then I use Locktite 5 min epoxy 8 oz. box available from Lowes to bond the strips to the hull.
You must take a wire drill brush around the window cut out and remove any coating around the window cut out making a 2" cleared square box around the window cut out where you will epoxy.
Clean this area with Acetone.
I use a wood strip also cut to size and multiple clamps to hold it into place as a press gluing the roughed up textured side of the f.r.p. strip to the hull first. This thickens the hull by 1/8 inch per layer. If I want a thicker layer I add another 4 strips only this time I will cut the sides long and the top short so the butt is in another location.
This builds any casement I need out of the hull and stiffens the window cut out allowing more bite for the screws into the plastic rather than the wood inside in the final assembly.
Some F.R.P. panel chemistries use S.M.C. plastic in them depending on manufacturers and its like pulling hens teeth to find out which. I use epoxies that are designed to handle multiple plastics on multiple materials because a straight modern epoxy resin for fiberglassing has trouble linking molecules with S.M.C. products.
The other product I use in this process is System Three Silvertip Marine epoxy or GelMagic, its bonds are broad enough to do the work I need with out a lot of knowledge needed about the chemistry regarding various plastics I am bonding.
The window will fit neatly and is way more leak proof than the original design.
I solved the leaks
in my refrigerator
vent area this way, I cut out the hull to the exact shape of my plastic vent bottom, inserted it from the bottom up into the hole I cut placing epoxy on the mating flange with the roof, "the flange normally reserved for the screws when these vents are placed from the top"
, once in place and the epoxy hardened I took an identical vent bottom (same model) and placed it over the one I epoxied into the hull like a glove, I hold that one in place with two side screws so no roof penetrations are made...(for looks I cut the flange off the top glove piece since its just the cap not a seal) that vent now can be replaced quickly and never needs sealing, it is permanent, and since the one underneath is its twin its a snug fit, the one underneath never seeing the light
of day will last forever.
If you have any other questions pm me and I can send pics of before and afters on the windows.
Happy Camping, Safe trails.
Harry, where did you find the FRP (fiberglass reinforced plastic) square casement? I am in the process of redoing my windows and that fiberglass shell is very flexible!