96 Trooper overheating

My Trooper will overheat when I pull a trailer, not a heavy trailer, just a small boat or cargo trailer. The heat guage needle will go all the way to the right, but the red light will not go on (does it have a red light?). When not pulling a trailer the needle is right in the middle of the guage where it should be. It has a clutch fan and I would like to know if there is a way to determine if it is functioning properly. I would like to put a flex fan on it but cannot find the right parts.

Any other ideas on what it could be, radiator, therostat, etc?

Start with pressure testing your radiator cap/system. I don't remember what the pressure should go to on your particular year but if the cap doesn't hold, say 13-16 PSI, your system will run hot. If your cap is good, pressure test the radiator and system. Again, what ever pressure your cap is rated for will be where your system needs to hold pressure to. If you don't have a bad cap or a leak, the other culprits are the fan clutch, water pump or the thermostat. If it's just running hot when towing, thermostat is LEAST likely the cause. If the thermo is bad, it will run hot anytime.

If your radiator and thermostat are original, this could be a big part of the problem. These items are probably worn out for sure.

ALWAYS use an OEM Isuzu thermostat, never aftermarket.