As a short term approach, lean the rv slightly toward the slide. I know on older rvs that tend to have aged weather stripping, it can help. The idea is to keep the water running away from the rv if any gets on top of the slide. Sometimes driving rain can get under the topper, and if the slide is tilted slightly toward the rv, water run toward the rv, and can get under the seals. The water will usually make its way to one side of the slide, or the other, and run down the sides of the slide. The water ends up getting the floor wet. The other alternative is pull in the slide if it is a driving rain.