You mentioned Yellowstone and Florida. That means that you travel on flat land and in the mountains. Since I live in Florida and travel mostly out West I can tell you that the mileage varies so much that there is no definitive answer that is accurate. Hills, wind, altitude, speed, loading, towing, and temperature all are factors that affect the mileage. You can drive over one road one day and have a tail wind and the next day, that same road could have a head wind. Since we travel a lot in the Great Plains, there is no such thing as a tail wind. It is always a head wind.
On average I would guess that overall, if you figure 7.0 MPG you would be safe, whether it is a diesel or gasoline powered coach. Some days you may get better, and some days worse. You may get better with some diesels over gas, altho I did not, but the extra cost of diesel more than offsets any extra mile or two of mileage.