Hiya,
When a Diesel pops a head gasket you will know it, they will drop that cylinder. How much coolant is it losing per hour of use? Is the radiator clear of chaffe in the fins? If the core area of the radiator is plugged, it will keep the coolant hot and tend to evap more water off. Water still evaporates from coolant, if the loss is minimal for the hours worked, you may be simply steaming it off.
Another thought, have you checked the ratio of glycol to water for the correct 60-40 mix? If you have been adding water only for the make-up, you may be making the issues worse by diluting the ratio and evaporating more off.
Have you checked the weep hole in the waterpump housing for wetness or rust deposits? I have seen them drip and clear everything on the way down to the ground. Most times they will only drip when the engine is running and hot.
If you can't find anything leaking, go get a pressure tester, pump it up to about 20PSI and see what happens to leak out.
Tom
Tom