Seals can break-down very quickly depending on intensity and duration of UV exposure (don't mention where you camp and how long it's open in a season), especially if cleaned with improper chemicals and not treated with protectorant. Unfortunately, you'll need a replacement.
My trailer sits outside all the time. I made the costly mistake of not keeping my slide in when parked and not in use. The sun turned my top seal into mush as described in first post. If you are handy you can replace it yourself. I found out the hard way that if you go to a dealer and buy the seals a 10 foot double seal(inside and outside seal in one piece) will cost you $300 - $500. Once I pulled the seal I discovered the two seals are not connected and all I had to replace was the outside seal. Too late to return the one I purchased so I split in half and only replaced the outside seals. My seals had a bumper seal which was seperate from the wiper seal. I had to replace both. You can buy these seals on line at a fraction of the cost of going to a dealer. Just need to make sure what you need to order. I'm now keeping my slide in when storing.
2012 Sabre 250RLUD
2011 GMC 1500 Sierra
Max Tow, 6.2, 3.73
Rod, Debbe 1 spoiled Chihuahua and Becky the cat