Needs 1/2 Gallon Coolant after sitting overnight

If you're not seeing any exterior leaks and/or puddles, and you're loosing 1/2 gallon every night, that coolant has to be going somewhere! Chances are its leaking internally either into the cylinders or the block itself. You need to get your vehicle to a service facility and have them find out where that coolant is going before any more damage is done.