Save your money for other "goodies"; you'll get more bang for your buck.
The frame of your trailer will rust, salt or no salt. No ifs, ands, or buts, even with the "rust prevention" that they will apply since there are areas that they can't reach with the spray (on top where the floor sits for example) and the rust will start in those areas and then proceed to undercut the sprayed on "rust preventer". Your frame is a ground path and the electicity flowing through it will lead to the formation of rust; one of the biggest issues with modern naval vessels, it isn't so much the salt water per se, but, huge amounts of electricity that flow through the decks and walls to "kick start" the rust process.
Without the spray and if you completely ignore the rust, the frame will still outlast the rest of the trailer. If you keep an eye on the frame and spot spray the worst areas with a can of Rust-o-leum once a year, you can do just as well and pocket the change. I've been RVing a long time and never had a frame rust through; those I-beams are tough and the surface rust may look bad (if you ignore it), but, a rust through will take a long, long time. A can of Rust-o-leum, a small wire brush, and an hour or two (my 5th wheel is 35' long...) and the rust is covered and blocked.
My 2 cents, your mileage may vary...
2 Cats; J-Lo and Ragamuffin :R
2014 Thor Tuscany 40RX DP
2011 Ram 2500 Longhorn CTD HO
2011 Keystone Cougar 318SAB (now gone)
2008 FunFinder X 210WBS (Sadly gone)