I don't have experience with both fuels in RVs but I do have it with light trucks. I currently have an '01 Dodge Ram 2500. My cousin has a late model Dodge Ram 2500 with the Cummins. Both are 4 wheel drive and mine is a heavy chassis while his is a regular 3/4 ton.
I can pull with him and keep up. Except for fuel stops. I get 11-13 mpg. He gets 18-20. I have 210,000 on the clock and he has 114,000. The premium up front for a Cummins is about $7500 as a lot of stuff has to be made heavier. Then again, mine is a heavy 8800 pound chassis and is basically a 1 ton truck with single rear wheels
He get's about 1 1/2 to 1 3/4 the mileage I get so the flag goes to the oil burner.
Otherwise, the flag goes to gas. If I run out of fuel, I put more in the tank and the intank pump picks it up right away. Run the Cummins out of fuel and you air lock the fuel lines and have to bleed them. If I need mechanical repair, just about any shop can handle it. The Cummins needs specialty repairs. When I fill up, I pump fuel available just about anywhere. When he fills up, he has to first find a deisel pump plus check his urea tank. It's called Bluetec fluid but is basically cow urine. In just about any weather, I start the truck and go. Really cold weather I'll let it idle a bit to warm up but it only needs that in really cold weather. Deisel you can't just start and go. It doesn't like cold weather. Gasoline is independent of temperature although different blends are done at different tomes of year. Deisel will gel on you in cold weather. I learned that the hard way when I drove a truck. You also have to watch for paraffin in your fuel as well as water. The alcohol in todays gas absorbs water and burns it in the engine so gas line freeze doesn't happen as often as it used to.
Oil changes for gas are like they always have been. Deisel goes a long er time but when you do change it, it's pretty pricey.
Lastly, in operation, deisel will give better grunt off the line as it has more torque. Climbing a grade at speed, it's not so much more as total horsepower is usually less with deisel than gas. That's why I can stay with my cousin, he'll pull away but on long grades, I catch up.
I think the advantage would go to deisel on a large pusher class A but on the smaller units, unless you plan to keep it for many years, gas is a more economical alternative. I can get a Cummins powered pickup similar to what I have but can't see paying the premium just to burn oil.