It seems to me that a lot of people (not necessarily you) have the idea that a longer class C is much harder to drive than a shorter one, and for that reason are scared of getting one that's all that long. Others worry about site availability for larger units--which is indeed occasionally a valid concern, but in many cases not a problem in practice. Still others suspect that it has a significant impact on fuel mileage, which is generally not the case to any great extent. For what are often mistaken reasons, they seek to compromise convenience and usability (tank sizes, etc.) when that's not really necessary.
The reality, in terms of driving, is that length makes very little difference on the road, and some difference when maneuvering around tight areas. The width and height are more noticeable concerns. I'd guess you may have some general awareness of these sorts of issues with your experience with a travel trailer.
As far as gas mileage, with my twenty year old class C based on the Ford E series platform, I average somewhere between 7.5 and 8 mpg, probably closer to 7.5 most of the time. New ones do a little bit better, maybe 1 mpg better, thanks to engine and especially transmission improvements. As with any motorhome, one's speed has a very significant impact on fuel mileage, since during highway travel most of the power is used in overcoming air resistance and that increases enormously with speed (I think it's approximately proportional to the speed cubed in rough terms).