I had a very similar problem with my 08 f450 wrecker. At first it was a minor leak but quickly turned into a half gallon a week!
Turns out mine was caused by what the dealer diagnosed as "cavitation". A small hole hade been burned inside the timing case cover by high pressure steam that caused coolant to go into the block, mixing with the engine oil.
Since I took truck the truck to "Qucik Oil Change" centers for regular oil changes, it was not immediately noticed that their was coolant in the oil.
My drivers, including myself, never saw the temperature gauge indicate a problem as we checked our fuids levels daily.
My problem originated from not having done the factory recommended modification on the coolant resovoir bottle where the dealer installed a "Tee" on the lower hose on the water bottle, bringing the top line of the bottle to the "tee".
Dealer stated that when your coolant bottle level falls below the top hose, no coolant is available to travel to where it needs to go, thereby creating the steam that created the cavitation burn hole in the timing case cover.
My truck first suuffered from the defective radiator crimp leak so we already had a leak issue but when the dealer replaced the radiator (under warranty) and pressure tested no leaks were immediately detectable.
I strongly suggest you speak with your dealer and ask about the radiator water bottle technical bulletin that ford has issued to see if that could be a cause of your problem.
Hope this helps...
