I believe this is a solid plexiglass window with a rubber gasket around the edge. Pretty sure Boler
, Lil Bigfoot
,... all use the same type of gasket. Post #3 here has a link to a Canadian supplier: http://www.fiberglassrv.com/forums/f...ler-68537.html
The critical measurements are (1) the thickness of the shell around the opening, and (2) the thickness of the plexiglass. It appears to be called a "locking gasket" so a Google search might turn up other suppliers.
A search of old threads should turn up some help with installation.