Not sure how long ago you flushed the coolant, but if there is truly no coolant leaking externally, then I can think of 3 potential problems:
1)When you replaced the coolant after flushing, you trapped air in your system and you are slowly "burping" the system and as a result, you end up being able to replace the burped air with additional coolant (the gurgling you referred to implies air in your cooling system
2) Your coolant is ending up in your crankcase (check the oil dipstick for milky engine oil
3) Your coolant is leaking via the heater core and ending up under the floormats in the cab of the truck (you would smell coolant in the cab if this was the case)




you truly can't find any leaking