rain water leak basement storage

Mine was a similar leak, rear basement compartment in a slideout. Turned out to be a seam where the metal support frame and fiberglass wall material met at the rear (towards the center of the coach laterally). Never saw it using the light in a compartment method, but did by using a targeted water spray (didn't take much pressure) to see where it was coming in. It got really wet when driving in the rain, not so much standing still, but did get some seepage even then - probably because the rain could get at it.

That said, I used a butyl caulk (smoothed and tucked into the seam) to seal the seam and it's held for several years now.