Have it scanned for codes- Just in case.
Try disconnecting the exhaust head-pipe from the exhaust manifolds. Then drive the SUV. If it runs A LOT better, then new CATS will probably be in order.
Another thing, maybe the front calipers are sticking, causing brake drag? Or even the rear e-brake (inside linnings) are dragging? You will have to pull the wheels off and look.
Also, you'd be surprised what a front end alignment, and/or a transmission flush/rear diff fluid change can do to MPG's....
Something to ponder.
Good luck.