Fluid Film works well but is expensive, I've used motor oil for decades with excellent results. Shoot (used or new) oil from a rustproofing gun or a one quart sandblast gun on the bottom of the vehicle and it will never rust. Park it in the gravel for the first few days as it will drip if you've gone wild. I used to drill the doors (like the old days) and shoot inside them also, it guarantees salt will never take hold. I've known guys to pop the door panels and put oily rags in the doors for the same purpose. A light spray, especially on fuel/brake lines guarantees them forever. If you shoot it every November you'll find you only need to do it a few years in a row and the oil seeps in every joint and never needs to be done again. When I sold my shop 13 years ago it included my 1979 GMC G20 van, which was shot several times with oil years back. The buyer of my shop sold it a couple of years ago, parked it in the front yard and it sold in 1 day at $2500 with very little rust.
Twin tank with switching valve? I've seen the switching valve on those trucks cause weird things. See if the guy is using it, or bypass it for a test......
Bruce Amacker's post in IDS Active Command was marked as the answer