The fuel mileage will depend on weight and and engine size. I would think it's safe to say that you will get a 2-3 more miles per gallon with a diesel of the same weight as a gasser. Generally, the diesel will be heavier anyway.
Our 40,000 lb., 425 hp Cummins, 2008 Monaco Dynasty has averaged 7.8 mpg over 69,000 miles. Maintenance for us is about once a year with an oil change and generator service. I have the oil changed every 10,000 miles and the genny every 250 hrs. The Aqua-Hot I have done every two years. The front wheel bearing greased every 30,000 miles.
House batteries lasted 6 yrs, I changed the tires at 57,000 miles that were 6.5 yrs. old. Chassis batteries lasted about 3 yrs., just bought two new ones.
I have/had things repaired as needed along the way.
MM.
