You guys are downsizing, so the only option you'll probably be comfortable with is a Sprinter class B. That's us.
I've been keeping good records on our new one with a phone app, and the results so far have been: 5411 miles, average mpg = 17.15, best has been 20.45, worst 14.85.
This is driving in Iowa, Kansas, Missouri, Oklahoma, Texas, Louisiana and the Arkansas mountains. Always right at or just below the speed limit, (68 on Interstates)
Sprinters are known for their mileage to go up after 10k+ miles. We're not there yet. Hope this helps