At 3500 miles and at an average speed of 65 mpg through southern California I5 and 101 getting 14 mpg.
Normal?
That's about right for a Class C with a "zero-time" engine. We just completed a 4500 +\- mile vacation in our View Profile starting at 3600 miles. No special effort to save fuel or focus on manual shifting. MPG ranged from 14something to 17something with two drivers and a little over 10k weight. I tossed the two readings in the 20s as out of sensible range.
MPG will improve slightly as miles increase... Don't expect miracles! 15/16 is about average .. Occasionally 17 on a really good day, IMHO.
Here's the issues: weight, air resistance (flat bits up front and the "Snuffy Smith" nose above the cab), air drag (big, fat, flat behind). Other factors are six tires vs four; road surfaces, grades, & winds. All these conspire to lower your MPG. The only thing fixable is to keep the weight down...
OTH, 14/17 mpg is about 20-30% better than other C class RV with non-Sprinter diesel drive trains and maybe 60/70% better than a gasser under similar conditions.
(Consolation... My neighbors Class A Tiffin "Allegro" w/ the Work Horse Chassis and the monster V10 gasser gets maybe 5mpg... down hill with a tailwind .... Usually less.)