It was pretty bad at first, maybe 6.5 mpg, but it has gotten better through the break in period and I am at about 8.5 now with 5700 miles on the OD.
The motor was a real dog when new and has gained power ever since. Even still I will probably have the motor turned up to 330hp/1000tq from the current 300/850 configuration. With that increase I think this setup would be perfect for any RV application, and many of the race guys that don't run major mountain grades all day.
The thing drives awesome! I easily run 70 - 75 all day long towing my jeep behind. Very stable in the wind as well. Love the 6 speed auto, makes back road driving a blast. I do have a weight issue where the drivers side is considerably heavier than the passenger side. With both slides, generator, batteries and the majority of the waste tanks all on the drivers side I am listing over on that side and notice it even more when cornering to the right. I need to get a side to side weight and think I have a possible solution depending how far off it is.
Driver comfort is awesome, leg room is not a issue (I am 5'11") love the big windshield. I thought the 14,600 lb front end would make for a rough ride, but it rides great. Weights are 11,450 front & 18,890 back and that was full fuel, loaded and with a about 70 gallons of water.
Definetly love this truck.