Consumption depends primarily on the size of your motorhome. Secondarily on your driving style and load. You will find a guideline in the following table:
Campervans: from 14 liters/100 km
Truck Camper: from 16 liters/100 km
Van Conversion: from 21 liters/100 km
Motorhomes C 20, C 22: from 24 liters/100 km
Motorhomes C 25, C 27: from 27 liters/100 km
Motorhomes C 29, C 32: from 30 liters/100 km
Class A Motorhomes : from 33 liters/100 km
Vehicles from McRent in Europe are basically equipped with a new turbo diesel, which allows you to save quite a bit on fuel consumption.