MPG's in vans doesn't seem to matter quite as much about weight(relatively), or aero(they aren't obviously), but more about gearing combined with weight and your right foot. I'll explain my experience! 
I've been daily driving vans for a couple decades now. I'm going to skip every single van, as most were the same, and just hit the highlights and lowlights.
1 - 1999 Chevy express 1500 conversion. 350 vortec, 4l80e, 3.55 rear gears, 235/75/15 tires. This was all stock with the conversion company fender flares, spoiler, and interior stuff. It was a rather heavy van. I needed to run 50 psi(the limit of the lt tires) or it was like rolling on a water balloon. I would get roughly 15ish around town. And the best on the highway was 21mpg. This was cruising across the flats of ohio. Most of the time in Pa was 19/20 highway.
2 - 2003 Chevy Express 1500 conversion. 5.3 ls, 4l80e, 3.55 gears, 235/75/15 tires. The best I ever got with this one was 19. It just didn't seem to cruise as easy on the highway as the 350 van...
3 - 2005 Chevy Express 3500 cargo. 6.0L ls, 4l80e, 4.10 gears, 265/75/16 E load tires. This was a beast! It was an empty cargo. So rather light for a van. This thing got 10 mpg. 10 uphill. 10 downhill. 10 towing. It didn't care. I may have squeaked 11 out of it once! I added a deep front air dam to it. It got 10! It was quiter inside. Significantly in fact. All the air rushing and swirling underneath was significant for noise. But not so much as to improve the mpg's at all. It was now quite enough, I noticed that my door seals were whistling with air. They got replaced.
Now here is were I say, MPG really seams to come down to gearing, more than aero.
4 - 2005 Chevy Express 3500 Long wheel base 15 passenger. 6.0L ls, 4l80e, 3.73 gears, 265/75/16 E load tires. 16 MPG's on the highway. A bit less quick off the line than the previous 3500. But it cruises so much easier. This van is heavier than the last one by a bit. But for all intents and purposes, its the same other than rear gears. I added the same front spoiler as the one before, and saw the same noise reduction, but no mpg improvements.
5 - 2012 GMC Savana 16ft Ex-Penske Box truck. 6.0L, 6l85e, 3.73 gears, 225/75/16 tires. The aero of a house. Best mpg is 14.2. The 6 speed transmission in this thing is magic. I never expected to get into double digits. To say i'm thrilled is an understatement.
These are just the highlights. I've owned another half dozen or more vans of various flavors. One was a ford. I didn't have it long, just didn't mesh with it well. It had the 5.4 triton. While it ran strong, it wouldn't do any better than 11 mpg's or so for me.
So from experience, the gearing is what made all the difference in mpg. And obviously driving style. I drive to get the best mileage possible. Not hypermiling, but grandpa style!
I've considered building a van for cross country style trips many times. If I did that today, I'd start with a 2010 and up 3500 express/savana. 6.0L engine, 6l80e trans, either 3.55 or 3.42 gears, 265/70/16(or equivalent 17's) tires(they come stock with 245 width, I prefer the stability of 265's). Add a front spoiler, make sure you have the smooth composite headlights(none of my vans do now, either 2005 or 2012), a better muffler(the stockers are really restrictive and terrible flowing), and a tune(I'm getting ready to do these mods to my box truck). I'm reasonably certain if you drive this combo conservatively it would get low 20's on the highway at least. Maybe even lower mid 20's. And for something thes size of a house, that's not to bad. I especially like my 155" wheel base 15 passenger. I removed most of the seats, so I can carry 5 people and almost everything else I own. My favorite part is, its all locked up and inside out of the weather.
Let us know whatever you buy/build!
Paul
