A van is a subject I’ve thought about a good bit. Vans are supremely utilitarian vehicles and like you many van owners love them. They are a bit of a challenge to improve MPG but you can get a decent percentage improvement because they are such egregious gas-hogs.
Some stuff you can do. First is aerodynamics.
If you have any luggage racks, lose them.
You can reduce frontal area by slamming the van. Your van is essentially built on a c-10 chassis and I thing both the front and rear ends are the same. There are plenty of slam kits available for C-10s of your vintage. Between 3 and 5 inches of slam leaves you with a roadable vehicle. I reduced my F350 4” in front and 6” in back and gained 1.0 MPG.
After slamming the van, an air dam begins to make sense. Either that or making the bottom more slippery.
Lose the 235s. Get 195s or 185s Get LRR tires. Air them up rock hard. I run 25% over the sidewall pressure rating. Get flat hub caps or Mooneyes. Follow up with fender skirts.
If you want real MPG, you’ll eventually have to do something with the back of your vehicle. The more you can reduce the trailing area the better but there is one proviso, you cannot make the reduction too sudden or you will cause the boundary layer to separate. My fastback cover is too steep at 17 degrees. A better angle is 12 to 14 degrees. That is a 1 in 5 slope. A 1 in 4 is fourteen degrees. The ultimate boattail is basjoos’ but his car is a lot smaller. You’d wind up with a boattail twenty feet long. Best compromise is to make a tail fairing that is 12 inches long and tapers in 3 inches from each side, top and bottom. Make sure the transition is smooth and gentle.
Mechanical stuff
Forget the Gear Vendors. Too expensive. Maybe $5000 installed.
A big improvement could be in the cards by an engine swap to a diesel. A 6.2 or 6.5 GM diesel is nearly a bolt-in for your GM van. Another one that will work is a Cummins 4BT3.9. Same power (125 HP) a bit heavier, but it is a better engine. If you are really adventurous and resourceful, a M-B 617.952 engine will fit easily and they make a fairly durable automatic. If you substitute in a diesel consult a performance transmission shop. Us Ford guys like Brain’s Truck Shop in Arkansas, but I’m sure there are equivalents for GM. Tell them you are building a very high-torque low RPM setup. You will rarely be revving over 1800 RPM and generally more like 1600. You need a tranny that can handle 400 ft-lb as a steady diet. Most automotive automatics cannot take this as they cannot move enough ATF to the cooler at low RPMs. This results in spectacular automatic transmission failures. Parts and ATF scattered down the road. Ask me how I know this. That is the reason I am a stick shift loyalist.
Check out which axle and ratio you have. I suspect you have a GM 12 bolt with a 3.73 ratio. Hit Rick’s Ring and Pinion
Randy's Ring & Pinion and see what R&P kits you can get. I’m sure you can get a 3.08 and pretty sure you can go down to 2.73. Even cheaper, raid a junk yard for the axle out of a 94-96 Buick Roadmaster Limited. Either way you would be reducing your RPM from 2000@70 MPH to 1450@70 MPH. This isn’t enough to lug the GM or Cummins engines. Those engines make staggering low-end torque.
The aero stuff is cheap and the mechanical stuff is expensive but they both work.