Repairing cracked black water tank

From the day we bought good old Mabel, she's had a leaking black water tank. A PO somewhere along the line did a very poor job of repairing what I think is a crack at the valve fitting with some fiberglass. The tank is empty and today was about peeling off that nasty old repair and trying not to get spattered by the black water still dripping from the tank. GAHHHHH!!!!! :E That was highly unpleasant.

I didn't examine the tank closely to find the casting marks, but all the research I have done so far says that RV black water tanks are usually made from polyethylene. That means that I can buy some polyethylene tubing from Home Depot (the clear Watts tubing is made from LDPE) and plastic weld the tank this weekend. I hope it will be less unpleasant than I think it will. I will report how it goes repairing the poop tank. :(

Jose