if this truck is an 04 , there is a good chance that the orange hose from the egr cooler to the oil cooler could be leaking causing a slight coolant loss, but it burns off before it drips , if this has been going on for a bit i would think the egr cooler would have taken a dump by now, if you get on top of the engine and look down in the valley of the engine , look and see if that connectore hose is orange or blue , if its orange thats the odl style and are bad to leak and cause the symptom you are having