Check gauge light come on and the truck move slow

Low or nil coolant
Broken or clogged coolant intake line

You give no details of vehicle. Have you checked the battery? it requires very little power to light LEDs, but considerably more to move analogue gauges.

Sounds like you have a bad sending unit in your gas tank. You will need to take it to a mechanic to get checked. This isn't really a do it yourself type repair since it involves removing the gas tank.

have a compression test done to check head gasket and cracked head/s. Flushing a radiator is a waste of time as blocked cores will not clear so have a flow test done on the radiator to gauge the number of blocked cores. Check the fins condition on the cores . If you have a viscous fan hub on the fan then replace it ( indications are that it overheats at lights ,when going slow or high rpms going slow but cools down when moving at speed). Check timing and air/fuel ratio as lean fuel mixture will cause overheating Check that the bottom radiator hose is not collapsing under high rpm blocking off the water flow.

Gas Filter Clogged ?

Check Fuel Pressure at motor with a gauge

Take the gauge on the road trip-tie to a wiper
so you can see it

Any check lights on?

Your loosing fuel or air metering data

Have Crank Sensor checked with a scope

Check Exhaust Converter

Check Fuel trim on road test

Check the alternator wiring and functionality.

your ground might be loose

It is most likely the sending unit in the fuel tank,but,check it first,if you unplug the connector to the fuel tank,the gauge should go past full.If you use a test light,and ground the clip on the end of the test light to a good,known,ground,and touch the right wire,it will light up dim(test light)and the gauge will pull down slow,to ,past the empty mark on the gauge,if it does this,the sending unit is bad,if it does not respond to any of this testing,then the gauge,or a connection to the gauge is bad.

1. check for low coolant
2. check for bad thermostat (open all the time ) it should open and close
3. look for cooling far work all the time

I had the same problem. My battery was going bad. When I replaced it with a new one, the fuel gauge worked fine.