I found your comment about wet carpet on an '09 Tour you used to own.
This is my exact problem on an '08 Ellipse. This has saved me replacing all the seals on my slide. I don't know how you figured it out but thank so much!!!

"Sure..with the slide extended touch the floor at the lower corners of the slide ( forward and aft) if they are there ...you should feel the well about 3 inches long or so just past the transition from rug to plain sub floor...less than an inch deep. Out side under the slide you can see the drain under the sub floor too...Hope this helps"