What controls how to make it not run so rich? sometimes it only gets 5 miles to a gallon of gas. Then it will run just fine for a while

The computer controls the fuel mixture. If your check engine light is not on then the ECU thinks everything is within specs and there really is nothing you can do. I used to own one of these gas hogs and 14 miles/gallon is all I could ever expect on the highway.

It can be several things. The engine automatically sets to a RICH mode for cold starts. It will stay in RICH until a temperature sensor warms up. There are sometimes 2 sensors with one for the dash and the other for the engine controls.
Next would be a faulty fuel pressure regulator keeping the pressure too high. Or dirty fuel injectors failing to close properly.
Finally, air leaks in the intake manifold will make the O2 sensors dump fuel to restore the fuel ratio. Check with water spray bottle on intake seams while engine is running.

I had a 98 Ford Explorer. It was a pig on gas since I bought it, and it was only 2. I kept this truck for 6 years and when I got rid of it and bought a full sized chevy, I thought about how much fuel I wasted in the past. This 2000 chevy 4x4 got double the mileage. I'd suggest selling it.

If the engine is running rich, you should have a check engine light on.
If you do, have the computer scanned for trouble codes.
It could be a fuel leak in the line or around the injectors too.

I would want to know what the fuel pressure is on the rail. Sounds like the fuel pressure regulator has failed, or the return line to the tank is blocked.

replace fuel filter and put more than 5 gallons, I had this same problem and now I can't let the gauge get below 5 gallons but the car runs fine now

Well, how much gas did you put into it? 1 gallon or 5 gallons, or more?
electronic level sensors sometimes are a couple of gallons off.

Roughly 200 miles for 10 gallons, or 20 miles per gallon. You should be getting 30MPG. Your fuel is going somewhere, if it is not coming out of the tailpipe it is leaking out of somewhere else. Check for fuel leaks.

file pump

the 1st thing u do is have the O2 sensors read to see if you are running rich, then go from there.