Steve B wrote:
...
I know to flush, fill with water, etc, to cut it.

TOTALLY unnecessary! Just cut it - there's no oxygen in the tank to
support combustion and no pressure to force anything but tiny amounts of
gas out. I have done it many times.

How about removing the mercaptan smell? ...

As others have said: bleach. Forget the potassium permanganate - you'll
have a devil of a time finding it and it's unnecessary (use a burn-in
fire to remove residual oil, etc & Bob's your uncle).

Not your uncle,
Bob