I've been thinking about the same project but the cost of windows
has slowed me down. I was thinking of replacing the side windows of our 83 mainly to update them to windows with rounded corners instead of the square corners. I think the rounded corner ones would help the appearance of the trailer since the door and front/rear windows have rounded corners.
Anyway on my 83 the rear window appears to be flat. If there's a curve to it, it's too small to see. Also there can't be much curve to it or the sliding window wouldn't work. On mine the lower sill is recessed maybe 1/2 inch in the middle and tapered to the edges to provide the flat plane mounting. The header and left/right sides are not recessed.
I would think that a window 2" longer (horizontally) would be ok but the vertical dimension should be close to the original. An inch or two larger in the vertical might give you curvature problems. If the new window has a gap on the left/right sides such that pulling the screws or rivets tight might bend the frame, then I would just make plastic shims cut to the shape of the end of the window and then use caulk to make it watertight/look nice.
Good luck and let us know how it works out.