2002 Mitsubishi Triton Overheating Problem

My 2.8litre turbo triton has a problem overheating. What happens is the temp gauge remains at about the half way mark which is normal. The overflow/expansion tank fills to the top and does not suck back in to the radiator. When the vehicle gets hot, the coolant in the radiator disappears. I assume it is boiling away. I have replaced the radiator, the thermostat, the heater coil, the radiator cap and the radiator coolant. The vehicle temp gauge doesnt go over halfway and this is the confusing part. I can drive 300km with no problems. When I stop and check the radiator, it requires abot 2.5 litres of water. There are no leaks anywhere, my belts are good and I have no problems with the water pump. There is no water in the engine oil, when the motor is running, there are no air bubbles in the radiator or overflow tank. Could it be the thermostat(even though I have put a new one in) or a blown head gasket? I dont want to pull the head of unless I have to as it will cost a fair bit.