Otozy:
Since no one is replying, I do have some questions for you.
1) Where is the area that you live or drive this truck in? I live in Wisconsin and because of the extensive amount of salt used as de-icer on roads throughout the winter, ANY vehicle driven in the winter for many years WILL succumb to rusting, even with frequent washing in automatic touchless tunnel car washes. What is happening to your truck is a common fender rusting problem.
2) As far as the running boards and the mounting brackets rusting out, this is common, too. Most running boards rust from the inside to the outside because of the outer coating, in your case chrome, breaking or cracking and allowing water and salt to seep inside.
The brackets may have been oxide-coated or painted, but this too will break down over time and rust. The ONLY way to "fix" this problem is to replace them. I say that from a safety standpoint because I`ve seen enough of them to know that they can break unexpectedly when stepped on getting into (ingress) or out of (egress) your truck by someone because of their rust-weakened condition.
3) Can you reduce or mitigate the rusting problem on your truck or any vehicle, for that matter??? YES, but the answer is obviously sarcastic:
DON`T drive the vehicle in the winter or if you do, drive it a climate where salt is not used, like southern California or far northern Canada. (Which is WHY they call me Captain Obvious!)
Your "fix" for the rusted truck fender lips of using bolt-on plastic fender flares is quite common here in Wisconsin, but it is a tell-tail sign the truck has seen and been driven in its share of Wisconsin winters.
As far as chemicals or sprays that you can put on rust to stop it from getting worse, that was discussed in this thread:
https://www.autopia.org/forums/car-detailing/192062-rust-corrosion.html?highlight=rust
BUT my conclusion is that once the rust starts and the vehicle is continued to be driven in the winter where salt is used on roads, rust WILL get worse as time goes on.