In the mid-10's my wife and I did several week long trips in a Dodge Caravan campervan conversion with the 3.6L V6. Keeping highway speeds to 65 mph we averaged 27 - 28 mpg in mixed driving. That is with a kitchen, fridge, extra house batteries and fresh / grey water tanks on board.
If you want a traditional full size van the Chevy Express with the 2.8L diesel will do want you want. Someone here on ecomodder has one and reports 30 mpg at 70 mph with loaded to 7500 lbs.
https://ecomodder.com/forum/showthre...mpg-36542.html