[RedDevil]

Welcome!

Judging by your own MPG assessment, you're doing quite well already, which shows you're willing to learn as otherwise you probably wouldn't get there doing deliveries in a hilly area and all

I can recommend maintaining a fuel log. It is a great motivator for saving fuel.
Air up the tires, that gives immediate improvement for free (not nitrogen: air is 80% nitrogen anyway).

Also an OBDII monitoring tool that shows instant MPG, like a ScanGauge, UltraGauge or ELM327 (clone) Bluetooth dongle with Torque on your smartphone will be invaluable; you can set those up to show other variables like engine temp, air intake temp, timing advance, engine load %, open/closed loop burning (the latter is more efficient), anything the ECU can throw at you.
And reset engine codes if needed, which saved me a few trips to the dealer...

Then of course reading the 100+ hypermiling tips and 65+ efficiency mod links on top of the page. Not all tips are practical, but every single one is food for thought.

The don'ts: It is a 22 year old car. Saving weight is fine, but not if it takes more than a little money or time. You are not likely getting another 100,000 miles out of it.

Propane injection works, but it's just replacing one fuel with another. There's a substantial risk and initial cost, and the gains are small at best. Again, I wouldn't do it in an old car - but then I would not in any car.

Ozone, HHO, Marvel Mystery oil, Fuel Shark, etc. are snake oil. All of them have been debunked well enough, never have these been proven to work in ordinary vehicles. Yours may be the exception, but you have better chances winning the lottery. Better put your effort in something that will work.

Then many of us are dreamers, investigators and experimentators, so whatever you do we'll appreciate the effort

But first things first: air up the tires, block part of the grill, start a fuel log, buy an UltraGauge, smooth out the hub caps, construct a boat tail, scratch that last one for now