We are newbies, and not trying to give you a hard time, but we are getting 15 mpg on diesel in our class C. I started driving up to 75 mph (where legal) and was getting 12 mpg. Now I keep it to 65.
(Shameless self-promotion) I have tried to drive for max mileage for decades now in all our vehicles. Very gentle acceleration, long anticipation of stopping (foot off the go-pedal), etc. Lots of info out there.
But maybe don't obsess about it like I do, just enjoy yourself. Isn't that what this is all about?