What do you do to the areas that you know have rust but cannot be seen or are not in reach to paint? Every RV I have had over the past 10 years the frames have rusted. Not one of them ever traveled on roadways that had been treated for snow/ice.
The travel trailers I had back in the sixties and seventies did not rust as badly as my latest RV's. I kept those trailers much longer than I have kept any of the RV's I have had in the past 10 year.
Unless it is an area or a structural member, like the hitch, which would be compromised by the rust, I just don't worry about the rust. That does not mean I like the rust; I just feel that certain things under the RV are going to rust no matter what I do.
Seems like rust on RV's is just another lack of quality that RV buyers have to accept. Remember the cars back in the 60's, 70,s and the 80's especially in the snow belt states? They were rust buckets within 3 to 5 years.
Ray