Wow!
There some crazy uniformed answers on this one. I HAVE a 6.4 in my Super C. See my picture in my signature. First of all, I would NEVER EVER EVER EVER own a gasoline powered RV. Now that I have said that, here is why.
I tow,
I drive cross country,
I drive in the mountains,
I boondock.
My 6.4 diesel is not good on mpg. I get between 8 and 9 mpg without my 10,000# trailer behind me. As soon as I hook up my trailer with Jeep and tools in tow, I drop down to about 6 mpg. I am unhappy about that but I am going to fix that. I will have to spend some money on the modifications but I will get a return on the money spent.
I also have 4:56 gears and 4 wheel drive, hence, part of my fuel economy issue. If you want to know what I am going to do to make my MPG's go up to about 10 mpg, first I started with an new cold air intake, the rest will come from tuning. Keep in mind, I built an '87 Jeep Wrangler that produces 300+ HP from the factory 6 cylinder engine. So, I know what I'm talking about. Some of you on this forum may not have the friends or engine building experience I have and that is understandable.
Back to the 6.7 fuel economy question.
Jerry, I am not sure what the Thor will get but the good folks out in Denver that built the Earth Roamer expedition vehicles base that RV on the Ford F550/6.7L diesel/4x4. I have the same chassis and setup as the Earth Roamer. They claim to be getting in the mid teens with 40" Tires. The Thor, with 2 wheel drive and no tuning should get low teens as a reasonable estimate. I hope this helps you out.