Have a 2013 28 BRS and had the same thing happen. Last year it was a small weep and we thought it was condensation in cold weather. This year it is much worse and happens evey time it rains.
Water under the couch goes both ways - toward the bedroom and under the sink, even leading to mildew under the sink.
I could duplicate with a hose spraying lightly for 1-2 mins around the window over the couch. Step by step troubleshooting led me to remove the window, but I saw no real evidence of a leak.
I resealed the window opening with plastic and waterproof tape, and sprayed below it gently and still had a major leak. Turns out it is the plastic trim (~ 4" plastic ) along the exterior bottom edge of the slide out. I am now trying to find the best way to seal it permanently.
Heartland appears to have sealed the same trim around the vertical sides, but not very well on the bottom edge.