You'll want to use a product that's intended for outdoor use, evne if the truck isn't kept out in the weather when not being used. Indoor rated finishes will not hold up, I learned that the hard way.
I have no experience with the products you've listed, but I have used Helmsman Spar urethane varnish, which is a marine grade varnish, with exellent results. Most folks use 3-4 coats, but after talking to my cousins husband (a professional woodworker), I went with 8 coats per his suggestion. He was also the one who told me which product to use.
