I don't own that rig but have family that do and have not experienced that yet. Things to check...when slide is out go look at all the black rubber seals around the slide looking for cuts/tears/missing seals. In addition get on a ladder to ensure that the top seal is intact. Can you tell me when the slide is out is there any separation of the rear wall near the bottom and watch when slide goes in if there are any gaps where side meets back wall. You should be able to treat the rubber gaskets with something like Aerospace 303 or other commercial rv products to ensure they stay flexible and keep moisture out. What year is your rig? If its under a warranty I would be contacting your dealer for solutions. Can you post any photos of the inside areas of concern and outside shots of the slide out area.
Daryll