I would simply drill a small hole at the end of the crack and keep my eye on it. The problem with welding aluminum is it really weakens any heat treating that was done to the material. That's one reason why aluminum welds do this, especially just at the edge of the weld. I really do think it's nothing to worry about, the welds around it appear as good as any of the others I've seen, and the ones that are on my camper. Now if what you really have is what I saw in your first post, then that's no beuno.