Realilistic mileage with a fifthwheel.

I have an older 5.9 Cummins and your unloaded mileage figures are pretty realistic, only a little optimistic. But 12.4 mpg dragging a huge 5th wheel is just stretching it a bit. Perhaps your odometer is off a bit for some reason. Figure if you're averaging 12.4 mpg, sometimes you're getting 13 or 14 mpg. Only time I've ever seen over 13 mpg pulling my 15,000 lb. toyhauler was coming east into Amarillo with a 40 mph tail wind. It averages about 10-11 mpg.

Edited to show east into Amarillo, not west. East is pretty much all downhill.