Hard to tell from the picture but, two things I noticed possibly. I only say this because I chased a leak recently. Do you have a gutter on that side? The other thing I noticed, it looks like the screws are not covered with anything.
I'd unscrew some of the screws and see if they are rusty. Chances are, you will find a section with rusty screws on that track. If they are rusty, that's where your leak has a high probability. They should have a vinyl cover of them that snaps in place. You can get it in rolls by the foot, usually 3/4 wide but it varies. It's probably at the top and running down behind the wall panels. Not sure exactly but, the bunks should have some wall support and if it has a beefed up plate on the studs the water could be collecting there behind the wall.
It's not fun to chase leaks for sure. In fact, I found mine last year and this year as soon as I have a free weekend, I'm going to redo the entire seam on both sides with eternabond. I'm going to pull the top gutter track off and put the eternabond behind there and roll it up over the edge of the roof. Then, I'll put all new hardware in sinking caulk in each hole as I put the screws in. Then, I'll finish it off with the vinyl covering over the screws.
Hope this gives you something to start with and hopefully some others will chime in with ideas too.
2012 Keystone Sprinter
Still sticking around for the good people and advice.