Help! Temporary repair to grey water tank.

You could coat what you have now with foam. I fixed a gas tank that way. Propped up a sheet of aluminum held about 1 inch away from the surface I was trying to repair. Sprayed foam in between the 2 surfaces and let it cure over night. The repair lasted just over a year, before the foam eventually pealed away. It may be worth doing in your case. I would put it right over the tape. With a long crack like that, the surface is probably not very stable. Foam spray and a sheet of plastic or aluminum should stabilize it, and may seal it off.

BTW: The place to get a replacement tank is http://www.tank-depot.com/product.aspx?id=163. I figure if I could find something to fit my unusual coach, you should have no problem finding one for yours.