$800! I don't personally know where you can get the seal, but I would be looking at any alternatives I could. I can't recall anything special about those seals when I had pop-up campers. There are lots of different types of seals available at home improvement stores. You might find something that will work.
2013 Jayco Jayfeather X20 E, 2008 GMC Sierra 1500 Crew Cab Z71 SLE