The anodization on the trailer should hold up really well on its own. I'd hit the following points with whatever corrosion blocker tickles your fancy:
Welds on the aluminum
Holes drilled through the aluminum
Cut ends of any beams
Hardware (all hardware, especially ground points)
Galvanized components
Anywhere that two components are touching (especially if they are dissimilar metals)
Won't keep it 100% perfect, but those should be the major offenders from a corrosion standpoint, unless I've forgotten some. The anodization on the aluminum beam surfaces usually seems to hold up well with normal washing unless you hit with an acid (or go through it, such as cutting/drilling/welding).
