[JRTJH]

There are about 8 or 10 black screws in the black vertical strip that the slide cable attach point is secured to. One of those screws may be loose/not sealed. Also, along the bottom of the slide, there is a white (on my trailer anyway) J molding strip. It's about 1" wide and wraps around the side to the bottom of the slide. On a few trailers, members have reported that strip as leaking. Once the water gets inside, it won't "flow up" but it will "wick up" into the sidewall. you might want to pull that strip and see if there's any evidence of water intrusion behind it.

ADDED: It may also be leaking at the bottom of the window and flowing along the aluminum crossmember from there to the outer edge of the slide wall and then flowing down the inside of the corner. I've had three windows in the last two slides I've owned that leaked. Seems that Keystone uses putty rather than butyl for window seals. I replaced all of mine with butyl and no leaks since then.... Your leak may not be from a window, but worth checking.....