Fuel consumption experience in an old V8 GM Van.
My build is a 2000 GMC Savana 1500 high top with the 5.7 liter Vortec motor that I picked up for $900. We don't live full time in ours but do go out for several weeks at a time. We're currently on our longest trip yet, from Austin TX all the way north, through the northern states and on to the PNW. When I got my van, everybody told me not to expect much better than 12-13mpg at best and that's about what I initially saw. I then decided to change to driving at the speed limit but maxing out at 65mph (62 if I'm going up and down mountain passes). Keeping in mind that I have a huge solar panel on my roof too.
Over the last few fill ups, I have managed to get 16.5+ mpg out of the old girl. This to me is amazing. On the first one I thought it was a mistake but I have seen it several times now.
Give it a try, you will be amazed!