First contact Trillium
235095 Range Road 284 Rocky View County, AB
They may be able to sell you the window gasket you need.
If they cannot help then contact these people. Vintage Trailer Supply - parts and supplies for vintage travel trailers and campers!
Tell them you have a window gasket that has shrunk, and send them this picture. Also tell them the age of your camper. Window styles may have changed.