If the wood is weather worn, I'd sand it first to get to "good wood". Then I'd use an exterior semi-opaque, oil based stain (black) like Cabot or similar that's used on barns. Apply the stain with a brush to all 6 sides of the boards (if removed). Keep applying it until it stops soaking into the wood. Once it stops soaking in, wipe off the wet stain sitting on the surface with the brush as you move to another area.
If you want to paint it, start by sanding it and then apply with a brush, a 50/50 mix of linseed oil and turpentine. Let that soak into the wood for a few weeks or months.
Then sand again to knock down the grain and paint with oil based primer. Let that dry and apply 2 or 3 coats of acrylic latex porch paint. It is made to be walked on and is extremely durable. Sherwin Williams / Duron or some other quality brand.
Do this to all 6 sides of the boards. It will last for years (or decades if the truck is stored indoors).
