How do I fix my Keystone slide, they won’t do it!

I think you're right and wrong about this issue. It probably didn't start leaking and separating in '04 or '05 but loosened recently. Yes, it could have been built better.
This could be the "poster child" for having a maintenance schedule that includes inspection of all areas of the exterior where water could enter. Had the water intrusion been dealt with in a timely manner, there would be a lot less repair work.