Our RV is stored outdoors 6 miles from the Pacific Ocean. Most of our RV destinations are at Beach campsites up and down the coast of California. Along the coast and inland where our RV is stored the mornings are foggy close to 50% of the days. This fog comes in from the ocean and carries salt with it.
I typically rinse the salt off after each trip to the coast (about 10 per year 4 to 14 day trips). The RV is washed an average of 2 times per year.
Our 5th wheel is 22 years old and only shows signs of surface rust on any unpainted metal. On our previous 16 year old TT the frame was beginning to show lots of rust, I used a wire brush to get rid of the surface rust and then rattle can painted it with rustoleum paint.
Sharing our experience... Our RVs have held up very well.