Thanks for the pictures. Outside of normal wear and tear, I don't see what else would cause that. I'm not familiar with these covers outside of a little research when they first came out. I'm not sure why they discontinued them either, besides there are so many applications, and the Solera only fit a few.
Have you tried to contact Lippert directly to see if they still have replacement sleeves? I can't find anything on their site anymore, so that may not be a good sign. Only other thing I can think of to do is remove the whole set up and put a traditional slide topper on it. With the sleeve on there, I'm not sure if there are wiper seals or not. Just not familiar with these things.
Another option that just crossed my mind. Try sending a private message to Wildcat Chris. He is one of the Wildcat factory reps on here. Go to "User CP" at the top of the forum. Once on that page, on the left you'll see "send new message". Click on that and type in his user name (Wildcat Chris) as the recipient and ask away.
Or, you can post your question about the sleeve in this section. It may take a bit, but usually he tries to answer all questions or concerns.
Ask the Wildcat factory...