John,
With one 15-pounder I did a midnight brisket, keeping a good low and slow under 250, for over 12 hours, in the 18.5. I had a full water pan (hot water).
With my three 15-pounders (in two weeks) I'm planning to do 12 - 15 hours, with a full water pan, at 225 - 250 degrees.
Also, I'm...