I was thinking something similar. It will make a difference. It certainly does on big rigs, and that's why they get full aero treatments. And the undercarriage is a big factor in overall drag.
I was thinking about doing something similar to semi-trailes; basically a barge boards between the body and roadway. I would make them semi-flexible to prevent damage, but also retractable. Instead of rigid mounts, they would be hinge-mounted with pneumatic cylinders providing the vertical support. Then I can retract them under the van on command, and with air cylinders they can be pushed up anyways by a curb, etc if I forget. I'd do something similar under the front bumper. This will also have the benefit of better cooling, since the under-side will be a low pressure area with the air-dam and barge-boards.
I don't think an undertray will work well with the vans because the the solid axles aren't flush with everything else down there.
My whole reasoning isn't necessarily better gas mileage. Its because of the new trailer. When its all done it'll be about 42' long and 13' tall and 12,000 pounds empty. That'll require more power. But instead of adding more power to the van, I could reduce power needs by improving the aerodynamics of both, and should still be able to pull 6% grades at 65+.