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I am now the proud owner of a 1966 ambassador double 28', and there is leakage around the back of trailer, what is the life expentance of the tank? is there a replacement available?
Before condeming the black tank, have you ruled out other possibilties? Since there isn't a gray water tank in the '66s (unless a previous owner retrofitted one), any water from the sinks or shower drains may be responsible for the moisture you are seeing. Another potential source of moisture at the rear could be a leaking Thetford dump valve - - the rebuild kits are readily available from most RV repair outlets. When I experienced problems with moisture dripping from the rear bellypan on my Overlander, it turned out to be a flexible coupling between the shower/tub "P" trap and the sewer drain assembly - - the solution was replacement of the flexible coupling with a similar part from the plumbing supply store. The one thing that has happened twice on my Overlander is damage having been done to the plumbing for the gray water that is in front of the dump valve - - on a rutted farm driveway, I managed to drag the exposed portion of the plumbing on the driveway cracking two of the pipes creating a leak.
If it does prove to be the black water tank that is leaking, it may be repairable. My Airstream dealer has been able to repair the black tanks in both my Overlander and Minuet when some minor damage was discovered. Should the tank prove not to be repairable, there are at least two companies that specialize in generic and custom tanks that might have a workable solution if a replacement is not longer available through Airstream and related parts sources. A search of the forums for "tanks" should turn up the vendors who offer custom and generic replacement tanks. Inland RV would likely be the most probable source for a new replacement tank if they are available.
Good luck with your investigation!