No it doesn't.
http://www.fueleconomy.gov/feg/findacar.htm
The Avalanche get better than 20. It is fuel injected and quite efficient for an SUV. But it surely is not “Super Size”. It is rather small in comparison to the HD-2500 or the HD-3500 Silverado models. (Which also get around 20.)
For the heavy work either of them do, they are quite fuel efficient.
Ever driven a car on Maui? 20 mpg might be possible is light throttle freeway cruising, but that does not happen on Maui. Slow and stop and go is the rule, so that boat is guzzling a lot of gas not even moving. He’ll be lucky to see over 10 mpg.