grey water tank leak

If its polyethylene it can be difficult to adhere to but not impossible. ABS is easier. Hobie Kayaks are polyethylene and I repaired some cracks in mine successfully. I first stop drilled the cracks, then sanded the cracks and surrounding surfaces. Then, very important, I flame treated them, which activates the surfaces and greatly improves adhesion. You just wave a propane torch over the area. Then I used gflex epoxy from West Systems. Good stuff, much more flexible than standard epoxy. My cracks were in high stress areas and stayed closed.

Tapping a hole is an interesting solution for a material which is very difficult to get an adhesive to stick to. You'd probably need at least a couple of threads engaging for it to hold. If you use NPT standard threads 1/2 inch thread pitch is  0.071 inches and 1 inch is 0.87 inch. So your probably want the tank wall thickness to be at least 0.15 inch thick for a half inch plug and 0.18 for a 1 inch plug. Anybody know how thick the tank walls are?