My former 93 Ram W250 came from Colorado and undercarriage was pretty rusty. I got my jitterbug sander, sanded complete undercarriage, hand painted mixture of Rustoleum gloss black and Rustoleum Kelly green, and in a couple days ENTIRE undercarriage protected. Had to grind bad metal in pax floorboard out, get some sheet metal and have it welded in. Undercarriage and body maintenance just as important as changing fluids regularly.
Sorry if I sound like a preacher... I learned from a friend in the military who did corrosion control on aircraft. I'm not even in a snow belt, but crawl under and inspect my vehicles every month, including checking fluids. Type A can work for you.
EDIT, and everytime I wash vehicles, I wash out frame, body and crevices underneath.
I believe your problem is very fixable if the rest of the truck is worth saving.
Cheers Again, Ron