We have pretty much the same engine. Our 5er tows at 10,000 pounds. Using the fuel economy gauge I can get 13.2 on flat roads at constant speed. It tanks once I hit the mountains but once I head back downhill the fuel economy recovers and I generally average 11.1 for the trip. From Portland, OR to Yellowstone I averaged 12.1 mpg one way. Stop and go traffic and headwinds stomp a mud hole in the fuel economy real quick. If you have the automatic you will find your sweet spot for towing will be 62-65 mph. If not already added, you will want an exhaust brake to tow that heavy with the 5.9 Cummins.
