New pressure treated wood has a higher copper content than the old copper arsenic based and must be used with stainless nails because the formula used corrodes steel. The new stuff significantly more corrosive -- 5 times more to common steel, according to American Wood Preservers Association. Due to the galvanic reaction between the copper and steel, fasteners should be stainless steel or copper. At the very least, you need to use a good grade of galvanized fasteners. I would think that there may be an issue with it sitting on your frame. I put some non-galvanized screws in some about 4 years ago and they turned to mush. Maybe some rubber or something between the wood and the truck to keep the wood from rubbing the paint off of the frame. Just a thought.
