I have a 2003 ram 1500 slt 4.7l. I have had to replace the entire cooling system back in 2011. Since then replacing the thremostat again. Now am having cooling issues. Running hot and the only way to run it is with the heater running full blast to blow off the heat from engine.

What could be the cause?

I have a 2003 ram 1500 slt 4.7l. i have had to replace the entire cooling system back in 2011. Since then replacing the thremostat again. Now am having cooling issues. Running hot and the only way to run it is with the heater running full blast to blow off the heat from engine.

What could be the cause?

During the life of the vehicle, how frequently has the coolant been replaced?
Corrosion can build up inside the engine assembly that will reduce coolant flow and reduce heat dissipation.
You say you've "replaced the entire cooling system." Does that include the water pump and radiator? Are the cooling fans operating as designed?