So basically OP needs to get rid of the big, heavy and non-aerodynamic box truck with an archaic pushrod engine and get a civic or similar? Or did I miss something?
OP: What is your MPG anyway? I'm trying to think of a modern vehicle for your needs that may give good fuel economy...I can't think of any.
I think a Ford Transit with the latest-tech. V6 gives mid teens MPG per the manufacturer. Around 14 city? I bet the real world is lower. Highway is a bit better it seems. I think that has to do with the fact they are unibody, and therefore, lighter than a frame-on-body equivalent. We know several families that have the Transits. One Dad told me they suck gas like crazy, but I didn't ask what the actual MPG was, or if he even knew.
My experience with the 8 lug Express vans is somewhere around 12 MPG, even the later ones. I drove a 2500 with a SBC 5.7 out of high school making window blind deliveries and drove it like I stole it (dumb me!) I recall 10-12 numbers. My neighbor is a paint contractor and his van has the 4.3/6 lug. I don't know what he gets, but probably the similar. We rented a 15 passenger once and drove to Georgia and I recall around the same MPG per the cluster reading, and loaded with people to the max.
Maybe the smaller turbo diesel engine equivalent full sized vans get much better MPG?
BTW, I don't think them Ford engineers got the Memo about NOT using pushrod engines on the latest 7.3 Godzilla engine for their newest F-250 thru F-650 medium duty line-up...