One thing I've found really works to prevent rust is axle grease. It's not very flattering appearance-wise, but it works, and works well.
I have a galvanized trailer that get submersed in saltwater all the time. Typically the nuts, bolts, lug nuts, and leaf springs, on these trailers rust like crazy after a year or so. I make a point of coating all of them in axle grease once a year. After 6 years of use I have zero rust on any bolt--they all look like new. Of course, you've gotta wipe off all the nasty, dirty, black grease to see how nice they look underneath the grease
I made a point of coating all the exposed metal and rusty welds underneath my toy hauler with axle grease. Since I don't soak my toy hauler in saltwater, my guess this one coat of grease will easily keep things rust-free for several years.