Take an hour and do the math. Compare 8 MPG with gas versus 11 MPG with a diesel at the current prices for diesel. Add in to the cost for the diesel the extra $9,000 for the purchase price and having to pay twice as much for an oil change and having to do it twice as often. See how many miles it takes to recover the $9,000.
Want to get great fuel economy buy a Mercedes diesel powered Roadtrek B class motorhome and you will get 25 MPG real world fuel economy. It takes fuel to produce the power to move a RV down the road and up hills and the greater the weight of the RV the more power, i.e. fuel it is going to take.
The advantage of diesel is greater torque when towing up a grade and better engine braking going downhill. But for that you pay a steep price.