[Caddylackn]

First, get rid of that passenger step, that is a real aero killer and causes a lot of turbulence.

The modular 5.4L and OD, is a good high torque truck motor with reasonable gas mileage for big vehicles. You can get a SCT tuner and get a custom tune with more ignition advance to run on only premium fuel, which will bump your gas mileage maybe 10%. With currently premium about 5% more than regular gas, this is a small savings. Long tube headers would be your best add on, if they make them for the van. They won't be cheap though $1000, but long tubes add quite a bit of torque through the cruising range and better mpg at cruise.

You can do what is called a J-mod to your trans. I assume you have the 4R70W. The J-mod removes a couple of springs in the control body that eliminates slow 1 to 2 and 2 to 3 shifts and also drill a few holes in the control body plate larger. So basically a shift kit that gives you a faster and harder 1 to 2 shift and a 2 to 3 shift.. You will save fuel accelerating since you minimize clutch slippage and gets you to cruising speed faster on the same amount of fuel.

Your mirrors are huge but IDK if there are better easier options for you.
If you could lower the vehicle 3", that would help, and add a front air dam off a suburban or other vehicle.

Then run about 5 psi less than the maximum tire pressure. Since you probably have 120 load rating tires, that would be 75 psi or so.