A bunch of PC7. (At Lowes, Home Depot)
It is the 2-part epoxy paste that you mix like play dough.
I would clean the area with rubbing alcohol then when it is dry I would make sure the PC7 is in the hole and a liberal amount around the hole on the outside then To insure it sticks and stays put I would screw an aluminum (Flashing) over the top of it.
It takes all night for this stuff to get hard.
Finally found tank repair kit at RV service in Binghamton. Just liked the one recommended in Frederick's link. Used two of them, crossed fiberglass tape. Looks great. Will let you know if it leaks.