All our motorhomes use Diesel.
Motorhomes are large vehicles that can be surprisingly economical on motorways, providing they are not trying to keep up with vehicles in the overtaking lanes. Fuel economy drops dramatically as speed rises.
The simple answer is that the way you drive really does affect the outcome. Anything from 21mpg to 28mpg is typical. One customer achieved 34mpg and took a photo of the readout to prove it, as they were convinced we wouldn’t believe them!