I humbly disagree. You can get 12.5 to 13.5 towing BUT it will depend on your truck's configuration. I built my truck to tow and it's not the usual truck sold off the lot at dealers.
F-350 SWR Lariat Ext. cab (not Crew), 2wd (not 4x4), 3.31 (not 3.55, 3.73, etc), All season tires (not AT), standard size:LT275/70x18E (not 35's or 10" wide), always use cetane booster ever tank (not ever once in a while or when I think of it), 5w-40 full syn oil (not 10w-30, 15w-40, or any dino oil), tow my 5th wheel at 65 mph (not 70,75 or 80) tires inflated to 65 front and 80 rear (not what ever they were in last August), trailer tires inflated to 75 (not what ever they were last time I checked?).
Not trying to be a turd but it all makes a difference and just because you can't get 13 mpg may or may not be your fault do to many variables in the big picture of how different our trucks are and how we treat and use them.
You're not being a turd, you're just in the minority.
Ford sells more CrewCab 4x4 Super Duty pick-ups than any other configuration and I'm guessing that 90% of the Forum's Members are driving "this" truck and those are the guys saying that they get 13 mpg pulling anything, 15-16 mpg in stop & go traffic and 20+ on the highway running at the rev limiter.
You're correct: Less weight and moving parts should yield better fuel economy (Super Cab versus Crew Cab and 4x2 versus 4x4). The 3.31 axle helps, but when you mate it to the 18" wheels & tires, the net result is the same RPMs as a truck running a 3.55 with 20" wheels & tires, so other than the weight of the wheels & tires themselves, there probably isn't much fuel savings. I also know for a fact that if you set up a vehicle in these particular configurations, that you'll take a pretty big HIT when it comes time to trade.
I'm not sure which Cetane Booster you're using, but there's a great article somewhere here on the forum and on the Internet comparing this products. The more popular ones are the Klean Diesel products, which have proved to be no better than cheap Walmart 2-cycle outboard motor oil. I prefer the Opti-Lube products because of their lubricating qualities. I've seen very little improvement in fuel economy, but my engines have all run quieter.
I've been driving Powerstrokes since 2001 and have logged close to 700,000 miles in 7.3, 6.0, 6.4 and now the 6.7 engines. I do all of my own maintenance and check every tank of fuel for mpg. In all of those miles, trying different weights, brands and synthetic oils, I haven't seen any increase in mpg with synthetic oil. On the pre-6.7 trucks, I settled in on either the Motorcraft or Rotella 15w40 dyno oils. I'm running the Rotella T5 semi-synthetic 10W30 in my new 6.7, mainly because it is readily available and I trust Rotella products.
Something that hasn't been mentioned in this thread is the winter fuel, which you never know when and where you're going to get it. I've seen winter fuel cost me as much as 3 mpg in all driving situations.