I have to say that some of those old motor homes were horrible on gas - with the 454 Chebbie being right at the top of the guzzler list - but a liter per km is horrible. Lets figure gas mileage in US so us Southerners can understand.

20L=5.3 gallons. 12miles/5.3 = 2.2 mpg!

First of all look at that carb. Stuck choke? Float full of gas? Dirt? Then next, the ignition. Is it advancing? Vacuum lines OK? Transmission shifting to drive? Spark plugs the right range? If all these are OK or can be repaired, and mileage doesn't improve much, then a compression check to get an idea of the internal condition is next.

Most of these engines had a Q-jet on them. While they are good carbs, not known for economy on 454's

Don't know how heavy/final gear ratio/what the frontal design is for that MH, but 7 to 9 mpg would be about normal for a carbed engine of that vintage in a MH. My 8000# F350 pulling my 11,000 5th wheel will vary from 7 to 10mpg, with normal about 9.