love my 06 mega cab cummins, bought it new and 100k later no major issues. i tow 10,500# of travel trailer every other weekend sometimes every weekend. jeep and trailer dont even know its there.. put a cap on it for the family trips, leave it stock, and run some fuel additive in it. my theory behind stock is it was engineered to do what it does and it does it good. im not saying an exhaust and maybe an intake but as far as chips and programmers go, i stay away. buy repair parts from a cummins dealer, they are half as much as the dodge stealer,(my dealers here buy the cummins parts from the same dealer as me for not much less).
mileage: empty at 1900 rpm i get around 20mpg 70mph
loaded with jeep at 1900rpm around mph i get around 16mpg
36 foot travel trailer weighing 10,500 loaded @1700 rpm about 65mph i get around 12-13mpg.
for me a diesel is the only way to go, only thing i would do different next time is go dually instead of SRW