For my last two trailers I 'Eternabond' the seam between the roof and the front cap. If this starts leaking, it will cause delamination in an awful hurray (so I have read on this forum).
In one of the pics above it shows the marker lights have an 'air' leak. I just make sure that the top and sides are well caulked and leave the bottom open for any drainage if needed. I may be off the mark here and willing to listen to alternate views of this proceedure. On my first trailer (an old one) I had to drill the bottom of the lense plastic for drainage as the fixture itself filled with water. Later learned that this was standard proceedure with these particualr fixtures.
Great thread. Can't wait to learn more.