RV generators use around 1.6 gallons to 2.9 gallons of fuel per hour of run time.
Diesel RV generators usually use less fuel than the gasoline powered RV generators do and they're more efficient to use as well.
So if you use RV generators while on the road then your fuel consumption will increase so you'll have to factor in the extra fuel to power the generator and will need to refuel the RV more often.
My RV generator is a gasoline generator and uses around 2 gallons of fuel per hour depending on the load on the generator.
The more electricity I use the more gasoline that the RV generator will use to provide that power.