Window replacement is really only a matter of matching the cutout in the trailer...BUT most newer windows have rounded corners, and I presume your Shasta has square (jalousie/louvered?) windows. Those are hard to find unless used.
But if all they need is new seals:
I'm not sure exactly which seals you're talking about, but if it's rubber molding of some kind, a lot of those are sort of generic and were used on lots of trailers. Many styles are still being made, too. Here's one website that has a real good chart showing profiles and dimensions- you might be able to match what you have here: RV weatherseals.
" Not every mind that wanders is lost. " With apologies toJ.R.R. Tolkien