I am a painter by trade and we specialize in commercial work where we run into old steel all the time. Ospho aka gem rust killer aka ready prep, etc is not going to keep the rust away permanently. Give it a few months or best case a few years and it will be back. If you want to go the route of the least work (leaving some rust) then i would reccomend you go with a product like PPG DTR (direct to rust) epoxy. This is not the best way but it will work better than treating it with ospho. The DTR epoxy does the same thing as ospho but it works while your painting it. The film thickness of the epoxy will be substantial and this is a good thing being that it would be applied on a trailer. If you plan to keep the trailer for several more years i would steer away from paint finshes and go galvanized. If you want a painted finish i would highly reccomend having it soda or bead blasted first, then use a good epoxy with urethane top coat (resists fading better than epoxy). We do gas and oil plants down near the coast and we always use a epoxy base with urethane top coat and they still look great to this day in the salt water enviroment. The key is the prep.

Carboline makes some excellent materials that would work well for this application. Sherwin williams and PPG both have excellent industrial materials as well. I can get you a steep discount on them too.