Most class C's built on the Ford E-series chassis or the equivalent Chevy chassis will get around 8 +/- 1 mpg, from what I've seen here, with the Chevy ones perhaps being a tiny bit ahead. One based on a diesel Sprinter platform will get somewhat better mileage generally (partly because of being a diesel, partly because they're usually smaller units).
The old Toyota pickup based class C's also get relatively good mileage, but are quite underpowered on the highway and have very little payload capacity...and, of course, are no longer new by any stretch of the imagination.