I know this does not address your issue, this applies to the Darco under the main slides, but it might give you some ideas.
The vinyl welder mentioned by Mike above sounds pretty good too. I have a split farm sprayer tank I have sprayed on several layers of flex seal. I'm not sure that will work in this situation.
Darco repair by Chuck. This is one of the best repairs I've seen.
https://www.montanaowners.com/forums...ad.php?t=83948
Chucks PDF on darco repair
https://www.montanaowners.com/forums...?do=file&id=64
Video by Chuck
https://youtu.be/EtgzzhXodlk