The 55gallon aluminum water tank in my San Juan 34 began weaping and within a couple of months got to the point that it would drain itself in about an hour. I cut a 12"x18" access port in the top and discovered that the entire interior of the tank was covered with aluminum oxide pox. After cleaning the tank I found a couple of sizable holes in the areas where loose oxide had piled up in the low spot. I epoxied aluminum patches in place with thickened non-aliphatic (sp?) epoxy and after cure I coated the entire interior. This fix is a year old and is working great.
Everybody (even the pros) advised I yank the tank but there is no way to do that without major deconstruction.