I have a similar issue. I discovered wet carpet and some swelling of the board on the inside front of my slide on the rear portion and damp mildewed carpet under the dinette seat. I removed the seat and let the carpet dry. Outside along the lower edge of the slide wall the interior paneling extended to the very edge of the lip of the trim piece and a 16'th of an inch below at one spot. It was exposed to the weather I think the rain water running down the side of the slide was being wicked up into the wall of the slide onto the carpet. The edge of the paneling was already crumbling away. I cleared as much of the bad away from the bottom edge and took a tube of geocel and sealed the wall edge up as best as I could. Waiting now to see if anymore water makes its way in.
Same problem from this link, slide out leak