We used Southern yellow pine on flatbed trailer floors as far back as I can remember, and that goes back to the 50's. We moped on linseed oil on a hot day as a preservative. I was always told to leave the bottom side natural to let any moisture out the bottom. Floors lasted many years being parked outside. I don't care for oak because it splits so bad. A couple of years ago I put Brazilian Ironwood on an 18' bumper hitch trailer and it looks like it did the day I installed it.