Checked the seal I think you are referring to on our '04 Designer 29RLTS, and it seems to be OK. Not sure why yours would be tearing, unless it got out of place somehow. I would email Jayco about it, and ask them where you can get a new one, as well as possible causes. Sorry I can't be of more help, but Jayco is usually very good at helping their owners.
2003 Ford F-350 V-10 Crew Cab 4WD Long Bed
2004 Jayco Designer Medallion 29 RLTS 5th wheel
Bill, Gayle, and Mindi (dachshund)
Retired at last !
Nothing is foolproof to a sufficiently talented fool.