Personally I would have to put my vote to a Dodge Astro or (as much as I dislike Fords) a Ford Aerostar.
"the Chevy Astro and the Ford Aerostar. Although no longer built, they are commonplace, cheaper to buy and roomy inside. They also get decent gas mileage. Some 'Vandwellers' forum posters have been getting 25 -26 MPG (highway) with their Astro's and I've been getting 23-24 MPG (highway) with my Aerostar."
The Urban Vandweller: Fuel Economy Part Four: Vehicle Choice
These models also have plenty room for improvement when it comes to MPG and aero mods.