I'm voting for either cracked head or blown head gasket... most likely between #2 and #3 cylinders for either. If you're losing that much coolant, its not leaking externally, and there is not a lake in the floorboard of your truck; you're pretty much left with an internal leak of some kind. The bubbles upon shut down can be a sign of this as well. Time to run a compression test.

Sean Strawmyer
Back and ready to rock...... crawl.

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