You must be wrong, my 6.7 4x4 (crew cab, long box) gets 16 unloaded aprox.
Get a gasser. The 6.2 is a great motor and after driving my fathers 6.2, the extra cost in fuel is worth it because they're cheaper to buy and not a SOB to maintain and fix. My 6.7 has already had turbo problems and it's in the shop right now because for some reason it's not making near as much power as since it came back. I'm not a diesel mechanic by any means so sorry about my crude terms but it only has 50,000 km on it.. Seems a bit early for a turbo failure.
New diesels are too complicated to be reliable. Say what you want to defend them and their resale value but what good is resale value if you have to spend thousands to fix them all the time? They cost more to buy but you sell them for less. I'd say dollar for dollar it makes much more sense to buy a gas one, they have similar resale value when compared to their original price. Sure diesels sell for more used but you also have to pay more for them used in the first place.
I wasn't a fan of the 6.2 the first time I drove it as it seemed to have to be revved to make power but after hauling a 40 foot travel trailer with both my 6.7 and my father's 6.2, I prefer the 6.2. It does it just as well and is a much simpler motor. My 6.7 isn't good on fuel at all loaded, it could be whatever in the hell is wrong with it but the mileage wasn't much worse with the 6.2 then it was my diesel.
2011 F-350 XLT, 56000km 6.7 powerstroke, 35 inch bfg all terrains