Search for threads on this site for slide-out leaks. Many people had the same problem I did. Mine leaked from behind the trim. With the slide-out retracted, look behind the upper trim piece where the slide-out roof meets the upper trim. That is where mine was leaking. When it rained water would get on the roof of the slide-out and seep under the seal because the roof was not 100% flat.
Easy fix, install Butyl tape over the 90 degree edge, where the slide-out roof meets the back of the upper trim piece. I put the tape over the entire 12 foot slid-out and down the sides too.
06 Hurricane 34FT WH W20 Chassis 8.1L 129K, Steersafe, Koni Shocks, DIY Trac Bar, Tri-Metric 2025RV Battery Monitor, 4-6V Batteries, Scan Gauge 2, Crossfires, 735W Solar Morningstar MPPT-60, KVH R5 In-Motion Sat, XM Radio, 07 Chevy Malibu Maxx Toad, Falcon 2, Brake Buddy, Escapee