As others have said the 6.0 can have its problems that have been talked about over and over
As for fuel mileage and tuners YES you can help with a tuner and the right tune... I have tried 3 different tuners Hypertech, Edge and now a SCT, hands down I like the SCT with a tow tunes from Gearhead automotive performance. The Hypertech had the best fuel mileage but they do not have a EGR delete tune and I would turn on the check engine light from time to time and put the truck in limp mode and had to pull over cycle the key and go on...what a pain in the butt
As for the Edge I did not like the trans program.
So all that being said with the SCT tuner and Gearheads studded tow tune I picked up 2.4 MPG when towing and when not towing I run Gearheads Extreme SRL tune
Also keep in mind I'm a overkill type when it comes to mods so your mileage may not be the same..
My list of Mods:
* ARP Head Studs
* Maxxforce Oil Cooler
* Blue Spring Fuel PSI Reg
* Updated HPOP Fitting
* High Flow Turbo Oil Supply and Drain Fittings
* Sinister EGR Delete
* AFE Intake and Air Filter
* Bullet Proof Diesel Water Pump
* Mishimoto Radiator
* Evans Coolant
* Mishimoto Intercooler
* Mishimoto Intercooler Piping
* MBRP 5in Exhaust
* 5in Aero Turbine Muffler
* Mag Hytec Front and Rear Diff Covers
* Mag Hytec Trans Pan
* 6.4 Banjo Bolts
* Boost, Exhaust Temp, Fuel PSI and Coolant PSI Gauges
* Coolant Filter
* RDS 60 Gal Transfer Fuel Tank
* Ride Rite Rear Air Bags
* B&W Companion 5th Wheel Hitch
* SCT Tuner with Gearhead Custom Tunes