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.
2007 Dodge Ram 3500 SRW 4x4 with 6MT
2003 TDI Beetle
2002 TDI Beetle
currently parked - 1996 Dodge 2500 Cummins Turbodiesel
Custom cab, auto, 3.55 gears