'05 3500 SLT Quadcab. 6 speed manual Cummins, Banks- intake, intercooler, monster exhuast, and six gun w/ speed loader programmer. At 60mph it's around 24mpg. Pulling a friends 5000lb boat I get about 15mpg at 60mph. Oh that's on level 5 on the programmer. Sounds like you V8 guys need to wake up to the smell of diesel!
Forgot to mention it's not a dually, but on a 4" lift with 37" BFG Mud Terrain's and 4wd.