Hi Rob -
sounds like the probe was the problem. However, 20 hours isn't completely out of the ordinary if you ended up cooking it mostly on the cooler side.
one other thing, in addition to all the good thoughts from everyone else... it does make a difference cooking on the top v. bottom grate. If you have a full water pan, the bottom grate will be cooler than the top grate. I'm not sure where you were taking your temperatures, but if you were doing it at the lid and, say, your temps were 225 at the lid. The temp at the bottom grate was likely more like 200-215. It would definitely take 20+ hours to get a butt up to 195 at 200 degrees of cooking.
As craig and jon mentioned, if you want to speed it up, run it around 250 to 275 and it will break through its plateau much, much faster.