I think you might think about advertising it in the Midwest area. These trailers are very popular here, all fresh water. Then replace it with a good aluminum trailer.
I do not know how long before you will have issues, but as you already know, steel trailers + salt water = Problems, at some point. I had one a few years ago, that I would take to Florida on occasion. I would spend allot of time before, treating it with a corrosion preventative, and taping over the drain holes in the trailer beams, to keep from getting water inside the beams, where you can not flush it out. I never had any issues, but it was allot of work. But, I was only in saltwater, once or twice a year. At the time nobody offered aluminum trailers in our area when you bought a new boat. There are plenty now. Your case is quite different, I would suggest considering a different option soon.