My black water tank developed a leak at the seam where the drain pipe joins the tank itself. That appears to be a weak point in the system.
I repaired mine many years ago using ABS cement and fiberglass cloth. Basically, I just cleaned off the surface where the leak was happening with a brush and water and then applied a layer of cement to the surface. While the cement was still wet, I pressed a layer of fiberglass cloth into it. After it dried, I repeated the process with another application of cement and a fiberglass patch. Finally, I coated the whole area with a final application of ABS cement. I think it's been at least 8 years since I did this and I've had no more leaks.
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