What is the fuel economy when towing?

We have found from extensive driving and reports from consumers that when towing a fifth wheel at 100 kph or slightly under on reasonable highways, you will likely use between 50 – 60% more fuel than doing the same without towing. In other words if your vehicle normally uses say 12 litres per 100 km then towing you will use about 18 – 19 litres. OR 8 km per litre will become about 5. This is particularly important when choosing a tow vehicle. Some of the modern common rail diesel engine 4X4 pickups today can get better than 10 litres per 100 km or about 30mpg gallon in the old terms. Allow 50% extra for towing and it becomes 15 litres per 100 km or about 19 mpg. I remember back in the 60s and early 70s you would be lucky to get that out of a car on its own let alone a 4X4 when towing.