What I find ironic is that in the 1970s and 80s, up to the early 1990s, there was a great selection of smaller (20 feet and shorter) rigs. Toyota Dolphins come to mind, getting 22 MPG.
European companies sells short "C"s with an immense amount of usable room and versatility. However, they have different vacation styles than here in the US, so having a rig where they park, extend an awning tent, and sit in a place for 2-3 weeks is not uncommon.
Not sure why we can't do that these days. The smallest class "C" available new is 24 feet (a Winnebago Trend). There is a smaller one that Cruise America uses, but it is a design unique to them.