Looks like there's at least hope I can avoid buying a new tank! Linda, yes, the pipe on the left is a standard 3" hub with 3" drain pipe glued into it. That 3" piece was too big diameter for tank outlet, and had been forced in with some caulking, causing the split. I cut the 3" pipe flush at the hub, probably a mistake since now I can't try WME's solution of grinding a bigger inside diameter on tank hub.
When I get vehicle back from mechanic next week, I'll attempt the patch with mesh, and look at my options. In retrospect, should have tried to grind outside diameter of 3" pipe before cutting.
Linda, do you have some leads on pipe parts and blade valve that would be better than Home Depot and the local RV store?