I use CorrosionX on the shell and fittings and Boeshield underneath on the frame, axles, stabilizers, etc.
Boeshield leaves an ugly coating of paraffin wax after its solvent dries. It is terrible, just awful to get it off the panel areas of the shell. I've used it though, by spraying some in the cap of the can, then taking a Q-Tip and applying to the rivets so it soaks into them, and onto all the cut edges and fittings (lights, locks, etc). Then after doing a section, wipe up runs and excess with a microfiber cloth. Not hard to do at all.
These treatments do wash and wear away. It's going to be a tough ticket keeping the trailer entirely corrosion free if its exposed to salt air, but you will soon see how your attempts are doing. Stay ahead of it, wash and apply treatment as needed. Preventive treatment is easy, extensive repairs are not.