I just made a round trip from Placerville,CA to Rogue River,OR. A trip of 380 miles each way. On the way up, I carried 30 gallons of fresh water. On the way home I drained all the water and made sure gray and black tanks were empty as well. I drove at exactly 57mph both ways and it took me exactly 8 hours for each direction including a fuel stop and a pit stop. I have a 2006 Dodge Diesel w/4 door 4x4 short bed and a 2012 Lance 855S. I had a 2002 Kawasaki KX250 Motocross motorcycle on the front receiver hitch, which weighs a total of 265 pounds and a trailer hauling a 2003 BMW Dakar F650GS Adventure motorcycle weighing a total of 800 pounds.
I got 14.8mpg both directions.
The exciting thing was; I went over the mountains at 57mph and still had plenty of peddle left if I were to try and drive faster.