My Esclade EXT gas milage

I have had my Esclade EXT for a few months now. I have somthing like 3300 miles on it. I wanted to see what kind of gas milage I could get if I drove it real mellow. I drove from home to the Lake house 126 miles one way. I filled up and reset my trip meter and MPG meter and jumped on the freeway. I drove real mellow 65 mph the whole way. I was some what shocked to see I got 21.3 MPG by the time I reached my lake house. I guess they were not kidding when it said 19 Mpg on the window sticker. My father has the regular smaller Esclade. He side he was getting 21.8 Mpg on the freeway from SF to Sac town. Im pretty impressed that a huge all wheel drive SUV like mine could get that kind of Mpg. My last rig a Ford Harley Davidson F-150 was getting 14.5 Mpg Freeway. It makes the Esclade look like a Prius