Fuel mileage

Soooo many factors in MPG. How you are loaded, wind, terrain, weight of the MH, the engine, etc etc. Gassers will get anywhere from 6-10. Some of the new short DP's can get double digits and FREDs (Front engine diesels) or B plus can get 20 or more. You would have to set your sites on a specific model and year coach to get anything close to an accurate estimate in MPG. Even then, all the factors can change it drastically. BTW your MPG seems really low. I would expect 8+ in your current rig.

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.

Depends on where you are (level/hills) and how fast you drive. We average around 8 mpg with our 8.3 Cummins but we only drive around 62 mph. I have driven faster in the past. At 75 mph it drops below 7 mpg. I usually do not get in a hurry. It is not worth the extra cost. Also generator usage will eat up a lot of fuel. When it is hot we do run the generator but only use the front A/C and it uses about a quart per hour = $1 per hour. We have ran both front and rear A/C as well as the hot water heater and it uses a half gallon per hour = $2 per hour. But at 62 mph you are using almost 8 gallons per hour = $32 per hour to go down the road.

It's not cheap. You have to like it. Some on here say if you have to worry about the cost of fuel then you don't need to buy a MH. I disagree, you can be smart and try to save where you can. Every little bit helps.

Your average was 5.5 in a 2014 28ft class C, seems low, do you drive fast ?

A class A will have about the same average, can average about 6 plus if you keep it under 65 mph. {gas}

My best mpg is 7 with a 33 foot class A gas, that is flat roads no wind however..