I second the recommendation for Alliance.
Perhaps I can add a little from my experience since I was blessed with a Beaver that leaked a lot.
Slide leaks seem to be from four different issues:
First, the slide itself is made with a sheet metal skin. There are seams and rivets that can leak and need to be resealed occasionally.
Second, there are two sets of primary seals on each slide. One set is for when it is extended and the other for when it is retracted. They are held in place by the glue that was on them at assembly. The seal can peal off, slip out of place or get hard with age. They typically are very difficult to replace.
Third, the alignment of the slide may be off or have slipped with time. That can cause the seals to not mate properly and allow gaps for water to get through.
Fourth, the water flow in a heavy rain may be more than the sealing system can handle. Our roofs are flat. Depending on the levelness, wind and rain rate a substantial amount of water can find its way to one spot. In my case that spot seems to be at the front of the bedroom slide that has never been perfectly leak free. I ended up using some non-standard methods to direct rain water away from the seals that I could not get to be leak free. Specifically, I added a glue on gutter above the slide and some strips of thin aluminium to the side walls. Between them, very little water gets to the marginally performing seals.
