If gauge is normal, then i would assume the thermostat is working also. I dont like the heater hoses, one hot, one cold idea. Im not sure there is a water valve on that truck. My 2001 5.4 has no valve. I think i would take both hoses off (heater) and connect garden hose to one end and flush thru good to see that if flows good... Also, turn the fan on very low speed, and the heater hose outlet should be about as hot as the inlet. Either no flow from heater pluggage, or some type of water valve.