Most of the gunk is just down in between the wood grain.
I would powerwash it, sand any places needed with a block sander, then reseal the wood. I like using marine varnish for durability outside. Look for one that is UV resistant. If you keep this truck inside you may want to use a penetrating oil. Tung oil is an example. This will give you the grain look.
Or you can go through the process of taking all the boards out and doing the same thing.
Neither you or this wood will last forever. Enjoy the truck as long as you own it.
