I need to drain the bad gas from my truck, pull the tank and fix a potential leak.

I put a box end wrench on it, but could not get it to budge. I don't want to strip it, and I really don't want to round it off. Givne that it is a gas tank, I don't want to use heat (my usual solution.) I have hit it with PB Balster, but it has had little effect.

Any ideas? I would like to get the bad gas out before it really turns into gum.

Zach