How many miles do you do expect to drive each year?
As realtors say, it's all about "Location! Location! Location!"
Will you mainly visit mountain towns and campgrounds?
Make lots of starts and stops? Drive over the speed limit? You'll burn more fuel and pay more at the pump.
Diesel engines have better fuel economy, but you'll pay more upfront for that savings.
Will you be a part-time RVer? Fuel costs are affected by seasons, holidays, and fuel shortages.
Neither! Mileage, destination, length of ownership, and maintenance are greater influences than fuel type.