My guess is a 12,000 pound wench would do it. I have never tried this, so its JUST a guess!
I realize I'm a little late to this conversation but if my wench weighed 12,000# I'd just ask her - very nicely mind you - but I'd just ask her if she would please pull me out. Since my wench weighs a lot less than that she's not an option.
No way in hell would I expect a 12,000 pound winch to pull a bus if it was really stuck. The real problem once you get any big rig thoroughly mired is suction. Once the whole belly of the unit is sitting on wet ground it takes a phenomenal amount of pull to break it free from the ground. As many have already said, the best cure is prevention because when you get serious about pulling you can pull really big pieces of iron apart. Pretty well every spring one of my drivers would get something really truly stuck. We could always count on something unexpected flying apart on the unit that had been stuck at some point after we got it back on the road. Don't get stuck.