Most of the RV owners that I know all whinge about the fuel they are guzzling.
Not all of 'em though!
The RV7 I fly will do 175 KTAS on 40 litres/hr. It will also happily do the same speed on 31 litres/hr, and that makes it comparable to a car in terms of litres/100km. (But several orders of magnitude more fun!)
The difference is merely operating technique. As Jaba says, a little education has enormous payoffs. Sorry if this has drifted off the original topic, but it may have a bearing on the final choice of build.