I have a 6.5 TD 30 ' motorhome which I bought new in 1994 (one of the first 6.5 front engine units built by Georgie Boy and perhaps the industry). Loaded it weighs around 15,000 pounds.
I just passed 150,000 miles.
I have accurately calculated mileage on four or five trips from MIchigan to Florida and back and I consistently get between 11.5 and 12 mpg. I drive 60-62 mph on the interstates. With three previous, slightly smaller gas motorhomes (one GM 454 TBI and two Dodge 440 carb) I consistently got 7.5 to 8 mpg. My oil consumption has not changed since the engine was broken in and runs a quart about every 1800 miles. The over the road performance feels about the same as the gas jobs.
I'd say if you can pull this off with the Duramax you'll be real happy with what you get.
Bill Carry