Agreed.
I had a 2006 Chevy Express 3500 Ext WB with a Duramax.
Great engine! It is a Generation 4 product.
Avg 18 mpg.
One huge difference, I never idle any engine. If you buy a pre-owned 2006 Chevy, or newer, the Driver Information Center in the instrument cluster will give you an LED readout showing the number of hours the engine has run. DO THE MATH. Don't buy one that has been idled. You'll be able to tell real quick by checking the truck's "In Service" date and calculating the total number of days the truck has been in service.
Idling is hard on EGR's. Not to mention the excessive wear it places on every part in and on the engine. If your hot, go sit inside the truck stop, or Burger King, or Panera Bread, etc an use their A/C. If you have to sleep during the day, buy slip over windows screens from skreenz.com and run a battery-powered fan. For heat, buy an Espar unit. Don't idle.
In a 2500 series van the additional 700 lbs of weight a Duramax block adds over the weight of the 6.0L gasser, puts you too close to the GVWR. Go with the 3500 series. Make sure it is an extended wheelbase. Ford does not change their WB in an extended van. I think they ride very rough. Why? Ford simply adds on more body behind the standard WB. You'll be tempted to leave the skid close to the back door and that makes the rear end very heavy. The front wheels bounce like a kangaroo.
GM extends the actual WB from 139" out to 155" and it rides much better than the Ford.
Headroom is always a problem in any conventional van.
I am 6'4" and the legroom was adequate for me. I recommend ordering one with the power seat option. Also get power-heated mirrors. And order without any glass in the rear and side doors. I also recommend the slider door instead of the side "swing out" doors.