I had the same problem on our TT which we recently traded in. The front cap oxidized within the first year despite machine buffing with compound, followed by machine polishing and then applying a high quality RV wax sealer on a quarterly basis. I finally before the warranty was up spoke with the dealer, who agreed under warranty to take care of it, but I suspect they only did as I did and buffed it as a month later the oxidation reared it's ugly head again. I finally gave up as we were planning to get a new rig and figured I would save the work until prior to trading in. In the interim I discovered a product called Recolor by Rustoleum that apparently has been used on semi truck cabs, very easy and quick to apply so I tried it just prior to trading in, did the whole cap in 45 minutes as opposed to 3+ hours of buffing and it looked as good as new. Since I traded it in I have no way of knowing how long it will last. Once the caps start oxidizing nothing you do will last. If we were to keep the TT long term, I would have bit the bullet and had the cap repainted properly.
