If you can find a Chevy Cargo Van to rent they ride as nice as most cars and mine gets just under 20MPG highway. Much better way to go than overloading a mini van. However the older Ford & Dodge cargo vans drive & ride more like a truck so avoid them for sure. I don't know about the newer Euro vans but I love my 3/4 ton Chevy for long trips.
We just got home from a 10 hour drive in my Chevy cargo van and my body feels better than if I took my wife's Kia mini van.
My trip was all highway and I averaged 18.6 MPG