[Yetti]

well as an Ex GM engine mechanic I have a couple of suggestions. one of the biggest over looked areas for consuption is through the transmission. the trans runs hot enough to boil off most water leaking into it from the radiator trans cooler. it may very well be the starting point for loss. check your trans and make sure the oil isn't murkey. if its not clear and clean its most likely the problem.
also if you suspect its a cracked head or leaking head gasket, remove the spark plugs after driving it while the engine is warm and the radiator has pressure from running. let it sit over night. in the morning use a piece of clear hose and see if you can suck any fluid out of the cylinders that has pooled on top of the pistons. something in the real small range like 1/8 dia hose works well for this. if you find fluid laying in the hole you have your culprit.
the alternative would be you use a endio scope to look in the holes for fluid.

last but not least have you checked your engine oil for coolant? this is an old trick. you climb under the truck and let it sit after loosening the drain plug just a tiny bit, just enough it doesn't seal tight. put a piece of clean white paper in a pan and put it under the truck where it can drip on it from the plug. I have seen it take days to yeild results. but once in a while you get lucky. if the spot on the paper is being made by water or coolant it will have a green or pink ring around the oil spot.