Because I am located in an area with many hourse enthusiasts, I have painted many horse trailers. Like you I have tried and looked at many options for the type of paint to use. I now use the third option you mentioned (Rustoleum Protective Enamel). It is a good, durable product for such applications. It is easy to apply but I usually spray with a 2.8 tip slightly reduced. I have found the coverage is good, the price is great and it has good rust protection. The draw backs I have found are the lack of available colors and slow drying time.