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...
I have a Class C on the Chevy chassis. I get about 9 mpg towing a car. The Ford chassis gets a little less MPG. If you are thinking of a travel trailer or a 5th wheel, it depends on the weight and aerodynamics.
The lite RVs probably get a little better gas mileage but may be lighter for a reason, not as well built.
AFAIK, newer RVs and tow vehicles do get a little better gas mileage but unless you are getting a small Class B, none of them have good gas mileage..