We pull a 30' bumper pull with a 2006 Duramax and have towed around the U.S. from one end to the other.
Before that we pulled a 25' bumper pull with a F150 Ford with the 5.0 engine.
We had that combination for many years, and put on a lot of miles.
With the gasser, we got about 8-9 MPG. Empty the truck got 14 MPG. It was underpowered for towing, but it worked.
With the diesel, we get about 11-12 MPG. The RV weighs about 8200 loaded for camping. The diesel has power to spare. But, when you get a head wind then your mileage with either one will take a hit. I've gotten as low as 9 MPG with the diesel on occasion.
But, you will never come out financially by swapping from gas to diesel.
Diesel costs more per gallon. Oil changes cost more. The diesel fuel filters are an added cost. The diesel has two batteries instead of one. If something goes wrong with the diesel, ChaChing, get out your checkbook.
I would stick with the gasser, and deliver Pizzas during the week to make the budget work...