There is an easy way to determine where the coolant is going;
If you are mechanically inclined, purchase an ultraviolet leak detection kit from your local parts store and use it. If not then take it to a competent shop and have them do the same.
These kits use a dye that you add to the coolant when cold, then you run the engine until its warm and the coolant circulates. After this, you use a small ultraviolet light (like a flashlight) to look around at all the cooling hoses, radiator, etc. Wherever the coolant has leaked out, the dye will leak with it, and glow brightly in the UV light. These things are sensitive, and will show even leaks you can't see otherwise.