Middle of the Aft Bunk
Look under the covers under the matteresses back there and you can see where the ballast tank ends. You can read about all sorts of methods to check the spot from baggies filled with water to peanut butter. I used vaseline on mine. After you find the spot glue it down with epoxy. Mine has been there for two years stuck with the vaseline and I may get to glueing it down soon, or not.