The 3500 Express with the 6.0 does not get horrible gas mileage. Towing my trailer, I get the best of three tow vehicles I owned. I also get 2 mpg better in normal use, and 2-3 mpg better in highway driving.
I took a 8 people in mine on a ski trip for the 4-H club this winter, and averaged 20 mpg over the 400+ miles. The reason for great mileage was we never went above 67 mph on the trip (the top speed of the older school bus) on the trip.
I also decided that the barebones (even in LT trim) van saving $40k over a new diesel truck is definitely worth it for my family. I can also afford a cheap small lease vehicle if I really want to save fuel even more.
