I would use real wood. As far as finishing is concerned I've always used five or six coats of a good marine varnish with a high UV filter. My theory is if it's good enough to withstand the elements in a marine environment it's probably going to work in a truck body. Another factor in this is ease of repairing/refinishing. If you have some exotic plastic that got dinged or scratched, can you duplicate the original finish? With spar varnish this isn't a problem.
