Need help with choosing model-Need high MPG...but I also need room to carry cargo.

A Chevy Caprice wagon or it's twin, Buick Roadmaster wagon would be worth checking out. They've got plenty of power and are a very solid RWD platform, not to mention having more cargo space than most newer SUV's, especially with the middle and rear seats folded down. They are both rated at 17/25 mpg, but it wouldn't be overly difficult to get much better than that with proper maintenance. They also have plenty of power with a 5.0L V8. One year even comes with the LT1 Corvette engine, I don't remember what year it is though. Regardless, you wouldn't have to worry too much about overheating or bogging down due to overloading.