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? 

If you are planning on snowbirding in AZ.. why not just go, and have it looked at there?

The parts from the USA issue is resolved, the RV mechanics are in high gear in snowbird areas, and most will come to you for a small fee, if any at all.

They are backlogged and it will be 2 to 3 weeks before they even look at it. The technician I talked with told me that attempts at sealing often result in further leakages and frustration. The likelihood is that I may need a new tank. The problem then becomes delivery time as shipments from the U.S. are unreliable and can take up to 10 weeks. Well, Snowbirding to Arizona is out for the foreseeable future, so repairs may have to wait until Spring.  I will update sometime in the future (if I live that long!!!). Cheers!