I'm guessing that you are looking for the vinyl inserts that go into the molding strips used to cover seams. Most Rv dealers sell these, though black can be hard to find anymore. Camping World sells it in 25 foot lengths. Is this what you seek?
Thanks Guvner. I have no idea if that's the same stuff Gary was talking about; The Camping World photo doesn't show a cross-section to be able to compare it.
BTW there's no need to ftp anything to this forum and, if you do, it doesn't work. I modified your link so now it opens just fine. Another way to handle photos is to attach them to messages using the Additional options button located below the message composition box (only visible while you're typing or modifying a message).