05 blazer

My son has an 05 Blazer with only 22,000 kilometers on it. He cannot get above 15 miles to the gallon on highway. Big engine in it but he does not drive it hard. Also his brakes are squealing all the time. Took it to the GM garage and they told him it was dust. Cleaned them out and a week later right back to the squealing. Anybody else with an 05 Blazer having the same problem.