Are you sure about 8.2? I have the yucon Denali XL and it has a 6.2 which is the biggest motor you can get in a Yukon. I think they used to offer a 250\3quarter ton version in the suburban but I don't think it had that big of a motor.
Anyway, I get about 14mpg round town, 10 if I use E85. On the road depends a lot on speed. Driving 65, I can average almost 20 mpg but 16-17 is more typical. As you can see from the previous post, the small displacement v8 does not get better fuel mileage in the real world. Don't be afraid of the 6L! The 250 version is geared lower and accounts for the poor fuel mileage.