Hate to say it, but you could be on the way to a blown head gasket. You could be pressurizing the cooling passages with cylinder pressure and passing the coolant out into the overflow bottle. I have had it happen to me in one of my cars and it was the HG. Do you push the truck hard? My car got me by for a little bit and didn't leak as long as I stayed out of the boost. As soon as I got on the boost though it was pushing coolant out. Good luck!