Our good friend that we travel with a lot had an 02 Executive 43' with the ISM500, basicly the same coach as the Navigator. Many times we have fueled up together then +-500 more miles fueled up again and would take within a gallon of each other, so about 7.5mpg. @65mph pulling near 5000#toads.
Dave, Miss Sue and Lucy the Scotty
00 Windsor, 40', Grand Cherokee Limited