I realized last night there was still some liquid left in the tanks.
I bled that off, and left the valves open overnight.

Note: Don't get that liquid on your skin. Not so much for the
obvious frost bite hazards, but because of that nasty ethyl mercaptain
odorant that is in the gas that will absorb into your skin. My wife
was not happy with me last night. My hands still wreak.

This morning, I chained the tank to a solid post, and used a load
binder to tighten it up so it wouldn't move. I lucked out in that I
managed to find a combination of links that allowed me to get the
chain very tight, but not to the point of crushing the tank.

I closed the valves, and used a propane torch to heat the bung until
it was too hot to touch.

I leaned on the handle of a 24" pipe wrench, and the valves backed out
easily. (they are RIGHT handed, by the way)

There is a thread seal dope on the threads that hardens as it cures.
As many people have told me, this makes the valves virtually
impossible to remove. Heat softens the dope, and makes it easy to
remove the valves.

Now, before I get flamed for applying heat to the tanks, let me
explain some logic here.

First, although the tanks were full of LP vapor, there was no liquid,
and there was no pressure in the tanks.

Second, the valves were closed, so no gas was escaping and available
to ignite.

Third, the inside of the tank contained 100% LP vapor. This is not
explosive. The explosive limits for LP gas is between 1% and 10% in
air. Had a fracture occurred, the flame would not have burned into
the tank. The flame would have only occurred where the escaping gas
was mixing with air.

I got advice from people saying to pressurize the tank with air, and
bleed it off a couple of times. This could have easily left me with
an explosive mixture inside the tank. Leaving it full of 100% LP
vapor was MUCH safer.

Fourth, this was a 100 pound LP tank. Heating the bung is not going
to heat the gas inside to any significant amount, and certainly not
enough to risk a pressure explosion. In fact, when the valve was
removed, there was barely any hiss of gas escaping.

Dave