There may be a problem with a seal but it may also be as simple as the way the RV is parked!
If you look at the slide and think about it, when we get too much water landing on the slide top, having it tilted just a bit to run off may be the easy fix. Rubber seals are a start but they are not truly sealed as much as diverting the water so if we encourage the water to flow away from the RV, it may be the better way to go.
I think of those seals as kind of like umbrellas? Good to keep some water off but not if it's coming a flood!