If there are no signs of leaks from the coolant tank and no water on the floor of the garage after driving, and yet water is being lost from the cooling system, have you checked your water pump? A failed pump can have greater leakage when the engine is operating than when parked. Also, you should check your engine oil to ensure water isn't getting into it through a blown head gasket. Admittedly, it does seem like a great deal of coolant to be lost through a head gasket, although I guess the second top up could be due to air locks in the system after loss over a significant period of time. Regardless, if you have been operating the vehicle with with air locks/bubbles in the cooling system, you run the risk of damaging the impellor in your water pump, adding to your misery. Even worse if you are running it with coolant in the oil. I agree with all of those who have suggested that you whatever it may be, you get it towed into a trusted mechanic immediately.
Brad
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