Road salts can be very corrosive to galvanized steel (as well as other protective coatings). When the salts are dry they are not very corrosive, but once water is added to the salts, the saltwater solution will aggressively attack the galvanized coating. If water is expected to accumulate on top of the salts and then seep down (which is reasonable to assume due to precipitation), it is reasonable to expect the galvanized coating to be consumed rather quickly in areas where saltwater solutions accumulate. If it is possible to have drain holes in the bed of the truck that allow water to flow out while still containing the road salts, that would help limit corrosion due to the wet road salts. The galvanized coating can also be coated with paint, powder coating, or epoxy to increase its corrosion protection.