I have a 1991 Isuzu Trooper V6. It still has a loss of power, I changed the fuel filter and the gas. We also do suspect the Tank is rusted due to the fact its been sitting for a while.

check the fuel pressure and flow , if the pressure is low replace the pump if flow is low replace the filter.

I guess you didnt follow the fuel line.?
what engine is there, Isuzu ran carbs last, until 1994
the fuel tank and all fuel lines match the engine.
some are gas others are diesel.
in what country, they sold them world wide and are not the same.
the line up is.
gen3 alone.
2.2 L 115 hp (85 kW) C22NE / 22LE I4
2.3 L 4ZD1 I4 (carb)
2.6 L 4ZE1 I4
2.5 L 4JA1-T
2.8 L 4JB1-T td I4
3.0 L 4JH1-T
3.1 L 4JG2-T td I4
3.1 L LG6 V6
3.2 L 6VD1 V6

on the USA car,
the book says, and I quote
"

The Pick-up (P'UP), Amigo and Rodeo models are equipped with the fuel filter located directly in front of the fuel tank, whereas on the Trooper/Trooper II models, the fuel filter is located along the inner side of the right frame rail, near the rear of the vehicle."

maybe yours is missing if not there....
next time post country and engine
gas/diese
and displacement."size" and cyl count

you have a bad fuel pressure regulator it is located on the fuel rail

check the fuel line pressure it should read 55lbs ,on the fuel injector rail is a tyre valve looking thing with a plastic cap ,connect a pressure gauge to this and see what you get

Depending on how good the guage is, when the yellow light comes on telling you that your fuel is low, you have approx 2 gallon left in the tank. It is never good to run on a tank that low due to heat buildup of the pump which causes pump failure. The gas acts as a coolant for the pump.

Tank could have rust.. Might want to get the gas more looked at - and maybe after that - replace the tank if allot of rust is present..

The metal pipes out from the tank rust drop it down and you will see the holes or put fuel hose right over them .

Check your fuel pump relay.

It will be located in the top of the fuel tank.

easy under the driver side need fuel tool from auto store. 30 min or srew driver push the clips in and pull. carefull lots of gas...