Like what alboy said, very common issue. The first thing I would check is along the top of the strip moulding that runs horizontally along the lower edge / end walls of the slide. Water runs down over this strip and acts as a drop edge. If there is one pin hole or void in the caulk along the top of this strip, the water will penetrate and soak the sub floor and carpet in that area. It happened to me once on a brand new rig. It was easy to fix and I was thankful to catch it before water damaged the sub floor decking.
Steve Rinker
Rochester, NY
2013 Keystone Sydney 340FBH 5th Wheel, 12,280 lbs loaded (scale)
2015.5 GMC Sierra Denali 3500, SRW, Duramax, CC, Payload 3,700 (sticker- not scaled yet)
Take my posts for what they are, opinions based on my own experiences.