If your using the old method of dividing the miles traveled by the # of gallons you pumped it is NOT an accurate measure. Things like heat, humidity, a slight incline at the pumps will all have an effect on the # of gallons you put into the tank. Just a difference of 1-2 gallons can throw your calculations out the window.
Only accurate way to keep track of mpg is to get a Scan Guage. It's hooked directly to the OBD port, and gets it's info directly from the computer. It will give you the mpg per tank, and also keep a running average over a period of time/miles. And, it's accurate.
Same guage will also keep track of engine temps, speed (mph). You will find that your speedo is off by as much as 3-5 mph when compared to the Scan Guage.
As for your mileage that you posted...it's actually in the "good" range for a V-10 class c.
As for me, I don't care a whit what my mileage is. We bought the rig to have fun, and experience the USA and what's out there.