Also, I'd recommend doing an oil analysis on the next oil change to see where the coolant is going if you don't have any external leaks. I've had coolant slowly disappearing over the past year or so and I have no external leaks either. My oil analysis from Blackstone said I have coolant leaking into my oil from some where along with high numbers of aluminum. I don't have any other symptoms with the coolant loss: no white steam, puking, external leaks ect. which could indicate EGR cooler issues or head gasket issues.
I've talk with a local Ford Fleet Diesel Tech who said my symptoms sound more like front cover cavitation (the front cover is made out of aluminum). Just another idea to look at if you don't see any coolant leaks. An Oil analysis is just a good thing to get period to see the overall health of your engine.
Front cover cavitation
A search of the topic will also give you much more information.
Not trying to scare anyone, just giving you more possible leads to fix and keep your truck running well...