My 90s-early 2000s Grand Caravans got decent highway mileage -probably 21. They had 15-inch wheels.
My 2010 Grand Caravan has 16-inch wheels and gets 23mpg at 70mph. I'm impressed, given the frame size and load capacity.
Last year, I picked up a low-mileage 2015 GC with power-everything. It has 17-inch wheels. Six hours on the interstates averaging 75mph with a heavy-ish load and it's clocking in at 25.5mpg. I'm really impressed.