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.