The wiper is just that, a wiper to clean moisture off of the slide when retracting. If you are seeing moisture inside only when retracting when wet then the wiper could be at fault. If you are getting moisture with the slide in or out then the wiper is not the issue. The actual seals are on the inside. If you retract the slid about a foot you can then run your hand around the seal from inside the Rv. If the seals appear to be good with no tears then the slide may may be out of adjustment and the seals not mating.