LOL no. I think gets between 12 in the mountains and 14 on flatland with the trailer. Double the frontal area, 8 x wetted area, roof of many disturbances.
If somebody tells you 21 mpg with a traditional 5th wheel, the proper response is to spray the current beverage out your nose.
1996 Dodge 2500 Cummins Turbodiesel
Custom cab, auto, 3.55 gears