Epifanes varnish. Used on boats for 90 years, you can't beat that. That's my two cents. Remember to do all the sides of the boards too.
However, if you use your truck for hauling stuff, and in the rain, put some crushed walnut shells in the second to last coat. Other wise, you'll fall down when you walk in the bed in the rain. It will also keep your load from shifting.
Now, if you use your truck for hauling, real hauling, I'd use a million coats of linseed oil. Tung oil is actually better at preventing water vapour transfer, but I like the way linseed oil smells. This way, when you are done shovelling gravel out of the back, you can wash it, and re-coat. You also won't fall down when you are walking around in the rain. Most importantly, your truck will have patina, and will smell awesome, like an old truck.
I realize full well though, that most trucks aren't used to haul stuff, and in-fact many aren't driven in the rain either, and most people want "shiny", not "workie".
