Acids will rapidly remove any galvanising leaving the bare steel exposed. You could try painting the surface with strong ammonia solution and then washing. This will dissolve any oxides that have formed on the surface.
Mike
If I was coming to buy a galvanised trailer of any age I would expect to see the condition you mention and would not view it as a negative, if I was you I would just give it a light wire brushing and then a spray of WD40 to add a bit of a sheen.
Metal polish like autosol will clean and take the dullness off. But it won't become light and shiny again - as mentioned, it'd look wierd if it was too shiny anyway.