I live less than 600' from the ocean with a prevailing onshore breeze & an almost constant sea breaking on the rocks. Rust & corrosion are a fact of life. Keeping material such as brass inside the house looking good is next to impossible.
RVs are as cheap as they are because of the quality of materials used & a lack of a decent paint job on the frames. They are held together with low quality steel fasteners.
Any RV exposed to salt air is going to deteriorate before its time. Rinsing it off is not going to help the damage being done underneath. If you keep your RV at a shoreside RV park that is windward to the prevailing wind, expect problems.
No, WD40 is not going to work. The stuff is good for temporary fixes for squeaks but it is not a rust discourager by any stretch of the imagination. For that you will need what saltwater boaters use.
My favourite weapon of choice is something called Boeshield. It was developed for the aviation industry & sprays on thin but turns into a waxy film. The idea was that this stuff sprayed onto landing gear stayed on even while hurtling down a wet runway at 150mph. Keeps RV steps moving quite freely with a couple sprays a year. WD40 is gone in a couple of days. For the frame I'd use a spray on undercoater or bedliner of some type.