Had same problem with my 22' full slide 2nd trip when rig was new. The original foam type rubber seal has open metal alligator type grip which will rust and crumble quickly when even a small amount of moisture gets in. Replaced under warranty and fell off again less than 2 years later. Looked for an encapsulated rubber seal on internet.
Found what I needed here : http://www.cleanseal.com No. ZML-6705. Clean Seal will send you samples. I had All Stars Rv in Louviers, Colo. do the install (4 hrs labor) after I ordered from ParKin Accessories (Clean Seal's vendor for small quantity purchases).
Installed in Dec. , 2010 and still in good shape plus seals much better than original seal. I had a lengthy posting in IRV2.Com at the time that you can search under dickiebob posts.