I started looking at diesel, but quit because they are hard to find, and when you do, they are much more expensive. I am looking at RVs from the 1990s to early 2000s. At some point along this...
If you get a large class A, the gas mileage is not going to be good. You are driving a large, heavy box down the road. Looking at what others have posted in the past, it looks like you will get between 7 and 9 mpg with a larger diesel pusher. The smaller and lighter the RV, the better your mileage can be. You will need to evaluate each one. However, as others said, driving around a home on wheels will take a considerable amount of fuel, no matter what you get.