The 2011 up Dodge and Chrysler would be close (30 mpg at 55, but probably more like 28 at 65), and there are already stripped cargo version under the Ram name. The 2011 up Honda with the 6 speed (only on Touring and above or standard as of 2014), would probably hit 30 mpg at 65, but it will set you back and then require gutting a nice interiored van. The 3.0 diesel vans will do pretty good but I bet wouldn't hit that 30 going that fast, especially if you also then spring for a high roof.
I think the 2011 up Grand Caravan C/V is you best bet. They sold those as cheap as $17,000 new, maybe less to big fleets like the Post Office.
Here is one with a rare factory tow package asking $5k. The tow package would be nice because you could also build an aerodynamic kitchen/extra gear trailer and just use the van for sleeping.
https://www.autotrader.com/cars-for-...ckType=listing