I am not sure if this will apply to the Rushmore. In my 05 PP slide I can get a little leak from the trim around the slide its self or around the windows. Also check to make sure the rubber seal around the slide is not cut and compressed to keep the water out. Even if you think a little crack in the calking is not that bad is a place the water can get in.
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George and Debi
2006 F250 PSD, 2005 PF30CK