New concerned and curious taco owner

Years ago, my dad had a work truck that had a diesel tank in the back that he used to fill his dozers and back hoes. That truck was pretty rusty after several years, except where diesel fuel had routinely spilled in the bed.
Hell, even WD-40 is better than nothing.
I'd clean the loose rust off, spray the heck out of it all with a couple of big cans of Rus Oleum, let it dry for a week and then Cosmoline or Fluid Film, or Dollar General rust preventative spray or something.
I've been doing that on my 2014 for years and the frame looks great.
My preferred method is Fluid Film, followed up with some lighter weight stuff to help the FF creep into the sandwiched sections of the frame. I also spray a little up into the weep holes in the doors, rockers and cab corners. Do it when it's hot out and that stuff really creeps well. I even get little trails of oil up the outside of my doors, which shows me where the weep holes are and you can bet it's creeping around inside the doors too.