Any good class B van conversion will probably get you better gas mileage than a larger class C.
Unless it's in really bad shape, or in need of a major overhaul.
Don't forget some of the other makes, like Leisure Travel, Pleasure Way, Great West,
Sportsmobile (although they're custom builds and you may not see many used ones out there),
and some older ones like Xplorer, Bigfoot (very rare), and some others, you can find them in
makes/models section on this site.
Then there are the Winnebago family of small RVs, as well as other big rig makers that have
added smaller B, B+, and smaller C models, to try to take advantage of the gas economy buyers.
There are Navions and Views on Sprinter chassis, both by Winnebago, and they probably get
decent mileage too. Most, if not all, Sprinters use diesel, btw.
So, it's not just a Roadtrek thing. But we like ours and it gets around 20 mpg to an Imperial
or Canadian sized gallon (4.5 litres). Works out to around 15mpg to the US gallon (3.8 litres).