Tedd, before you install the planks dip them in the wood preservative to get it deeper into the wood. You can make a "dip tank" out of heavy poly (6mil) and some 6x6 timbers. Drape the poly over the 6x6 timbers so that it forms a trough, you could build a "tank" out of 2x6 lumber too, in any case make it long enough that the trailer planks fit. Fill (well, not exactly full as it will overflow when you dip the planks) the trough with the preservative and then dip the planks into the preservative and let them soak for several minutes to allow the preservative to get deep into the wood. You'll need a rack, the traler frame could work, to place the planks on to dry. Be sure to place a piece of poly under it to catch the drips. We did this 10 or 15 years ago when we replaced the floor in my parent's horse trailer and it is still in excellent condition. We may have used linseed oil or it might have been CWF preservative I forget what it was as my father bought the stuff. Be sure to wear protective gear, safety glasses, chemical resistant gloves (nitrile gloves are good) and perhaps even a half face respirator.
Or you could just paint the wood.
Or you could just paint the wood.