Hopefully you have a different setup. But, on our 06 Surf Side, we have the same problem and our usual service shop, and the local dealer cannot find the leak. Of course, the fact that National chose to install the tanks and build around them does not help. I honestly do not think there is access to the tank without a lot of tearing apart, either from above or below. So, at this point we are trying to live with it. I asked for advice here and on another forum and so far - nothing has helped.
Good luck - will be following with interest.