Were looking to get my mother an 05 4x4 envoy 4.2 liter, and im just curious about the mpg,s

I used to have an 03 Trailblazer with the 4.2L. I really liked the DOHC 4.2L I6. Very smooth and efficient engine that will last forever. Though it does seem to drink the fuel. I would average around 15-16mpg combined city and highway. Towing my 4500lb boat and trailer from Richmond to Morgantown I would get around 10-13mpg. Sum up, it's a great engine in a heavy pig.