From my experience. In mid 2010 I built 3 Harbor Freight trailers. All used same dimension lumber, 3/4 plywood deck and 1x4 sides.
1. trailer one; PT & painted. Condition today=solid
2. trailer two; PT unpainted. Condition today=solid
3. Trailer three; untreated but painted plywood. Condition today= signs of serious rot.
All 3 trailers are kept out in the weather. Painting deck is useless in my case because of what I haul would destroy(rub-off) the paint anyway. Low cost and after 6 years still good its a no brainer to me.