My 2014 suburban regularly gets 18+, sometimes low 20s on long drives. Suprisingly good.The Yukon XL is not bad for what it is, 19-21MPG whether it is empty or full, box on top or not. The Alltrak on the other hand, to Sun Valley and back, 30 MPG. With the box on to Colorado? 25MPG. Thats just terrible for a smaller car.
I just rented a Nissan Armada in Utah; it was a gas hog. I filled it 3x over two weeks (I think 20 gal tank). Never drove farther than Snowbasin out of Cottonwood Heights. Never really liked that vehicle in the first place, but that put a nail in it for ever wondering about it again.