Hello Jullien,
First contact
Trillium. Address:
Team
Trillium Manufacturing
235095 Range Road 284 Rocky View County, AB
Phone: 1-587-998-3929
Email:
trilliumtrailers@gmail.com
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.