WOW! 10 MPG? I must have a very special truck. DW and I were so impressed with its ability to do ALL that the advertising claimed that we named her GLORIA because she is glorious. Other than tires we have not had any trouble with her. She now has 132,000+ miles. I finally changed the brakes at 118,000 miles, and serviced the cooling system at the same time.
We have taken several long trips (8,000+ miles) and numerous short trips and always averaged 11.9 to 12.2 mpg. 62 to 63 mph is our sweet spot. Without trailer 18 to 21 mpg (Depending on A/C running).
2007.5 chevy 2500hd 6.6 dmax (LMM engine) crew cab with tow package. changed the fuel tank to 52 gal.
2008 Montego Bay by KZ. 37 feet long 13'6" tall and 14,100 lbs ready for camping.
Combined weight (truck and trailer) 21,400 lbs (scaled).
IMHO I feel once you find your sweet spot your mileage should improve.
Ron
