You can see the bead of caulk (dirty now instead of white) that runs along the bottom of the aluminum skin and above the green military trailer chassis.

That bead of caulk is starting to give way. The expansion and contraction of the aluminum is probably the reason for the failure. Not long after this photo I fixed it temporarily with Gorilla tape so that water wouldn’t work its way under the skin.
I could easily scrape off the old Sikaflex and make a new bead, which would last another few years. I imagine this kind of maintenance is par for the course if you own a home built trailer.
Before I go ahead with that, would anyone recommend a more permanent solution? If so, what would you do to make these seams waterproof?