Yearly oil coating helps slow it down. But you can’t kill those metal mites.
Rust is extremely hard to stop but Fluid Film and similar type coatings (NH Oil, Woolwax, etc) seem to be the best way to slow it that I've found.
I’ll be curious to know how the fluid film works for you. I’ve been reading some great reviews about a product called surface shield. Same company that makes PB blaster and I’m really interested in trying it out but for now I’ll just keep getting the oil spray. Ill Just have to keep a close eye on the dually frame after seeing how quickly the frame in my 3/4 ton went down hill. I probably won’t drive the dually in the winter if all possible like you plan on doing.
I've used FF in the past and it works well. It will wash off around the wheel well areas after a bit but the majority of it won't wash off. I used it on my old F150 and it kept that thing from getting worse and I drove it through a couple winters (there is a thread on that truck as well). I also use it on my tractor and all the snow plow components to keep the snow and ice from sticking and it keeps it from getting any rustier.
I've seen some ads and YouTube videos about PB Surface Shield and now that I'm getting to the bottom of my 5 gallon bucket of Fluid Film I was considering buying a bucket of Surface Shield for next year. From what I understand, while Fluid Film is straight lanolin, Surface Shield has lanolin and petroleum jelly to keep it from washing away. The biggest drawback is the price; Surface Shield is around 75% more expensive than Fluid Film which is hard to justify. If the price comes down, I'll give it a shot.
I'll keep driving it regularly until they start throwing salt down. Once salt is down I'll try to keep it to once every week or two and only when the roads are dry. We haven't come close to seeing snow yet although I imagine it's not far off.