This should come without saying but make sure that it doesn't have any pressure in the tank before you start. A really big pipe wrench or a pipe wrench and a cheater pipe is best way to get it out.
I used a pipe wrench with a 6' cheater pipe to break them loose. I'm glad it was still strapped to the trailer. That crap was crazy tight.
Do you know how to besure it don't have any pressure in it??
This forum is great. This is the type of tank I have so the valves are set in behind the shroud there. I think I'm going to need a socket with extension. Cheater pipe looks like the way to go though!