I did not remove the screw cover plastic trim piece on the gutter rail when I did my leak test. I just looked for bubbles coming out from anywhere on the trim piece, which were not evident. I can see that a leak from the plastic screw cover could be a moving target since it would not be coming directly out of the trailer shell at a fixed point.
Looks like my next step is to remove the trim cover and and do another pressure test of the rail.
I am told there is supposed to be putty behind the rail and that the screws pull the rail tight against the putty to create a seal. I'm not sure how to tell if the putty is there or not. It would appear that the only way water could infiltrate at this joint is through the screw holes themselves. Insight from anyone familiar with the construction of this screw joint would be appreciated.