The natural impulse is to attack the lowest part of a dent. When dollying-out dents, the method is to start at the perimeter, then work your way around more or less spiraling-in toward the center, as the metal gets straighter. The final step is heating the metal, then quenching it rapidly to shrink the area...hopefully...eliminating the stretch created by the crash.
Might try using a few more studs, in alternating offset pattern. If you have an assistant who knows how to handle a torch (propane is fine for this) heating the metal near the edges of the dent (keeping the flame directed away from the deepest part) will soften the metal, might make things easier.
If a tank surface cannot be completely straightened, I leave a shallow depression that can be leaded, leveled, then scratch-filled.