overheating...2010 with 330k miles..47 to 50 mpg still..

When draining the system of coolant..I opened the pitcock..on the radiator and nothing came out...330,000 miles...I used water hose to see if I could move some of the accumulated trash out of the bottom...Was partially successful...so went to service mode after replacing pump and thermostat....added coolant til fans came on...Driving home after a few weeks of local driving to test the repair...I got the overheat lite...parked and checked and noticed that the fans were not on... I noticed early on that the cabin heat was very mild...
so where is the fan thermostat the one at the top and rear of the compartment..near where the hoses lead into the cabin? I opened the air purge valve near there when filling initially...when it overheated that caused the pump and thermostat replacement...I opened the filler cap too soon and the coolant came out and left a chalky white film on that side of the compartment...Engine runs fine , oil clean..no sign of head gasket leak..sooo should I pull the radiator and replace or try to flush or is that an additive ? Or something else...Tnx