I must admit, I am surprised.
rmay635703, I'm glad you're from around here so you'll understand the locations I mention in this post. I put coroplast under my van and tried neutral coasting as much as possible. Easier said than done. In the country I was doing steady speeds, but I'm still learning in-town how long it takes the van to slow down. It spends a LOT of time at 30mph when coasting I now know...
Before Christmas, I worked in Oshkosh. I would drive from just south of Fond du Lac to Oshkosh daily, taking Highway 45. Once a week for a month I did a "per trip" test, driving from the gas station, home, then work, then back to the station. I always go to the same station, same pump. I determined that a single average round trip cost $12.50, and at a 57-mile roundtrip, that's 11.76mpg at the fuel prices then.
Between today and yesterday, I put coroplast under my van going from the front bumper to the lower control arms, coasted in neutral whenever I could, and only took Highway 45 at 50mph instead of 55. It wasn't a direct "roundtrip test", but instead I drove home from the pump last night, went to work this morning, put the coroplast under my van, then drove up to Oshkosh and back, stopping at the pump on the way back. Even though I had to run my air conditioning the entire time to keep the windows from fogging up, I quickly punched in my calculator... 75.6 miles / 5.036 gallons... 15.008mpg!
Obviously I can't verify whether it's the neutral coasting in-town or the coroplast on Highway 45... but something is making a huge difference here!
Phone camera sucks, but here's a picture of the coroplast as installed under the van... going from the front bumper to halfway under the engine crossbar, with the corners under the lower control arms
So while there's still room for improvement, especially with how I had the a/c running throughout the majority of the trips.... I must admit, everyone's advice (especially rmay635703's) is really helping out here!