If the water is only coming in after a rain, it's probably the slide seals. kenhwy61 is right about the leaks in the pex hoses behide the shower. Try removing the panel at the outside shower/dump valve compartment and taking a look. Only 4 screws and a pull. LOTS of hoses back there and you may find your leak. I did!
Also, don't know where to post it, but here is another FR recall notice.
2013 Silverado 2500 HD Duramax/Allison Tuned
Aviator Wright Flyer #1919
"First In, Last Out". 11B4Y
DAV, MOPH, VFW, 5/7 Cav,1/8 Cav Assoc.
11th ABN Pathfinder Co. N7MIP