The EGR is inside your radiator, if it is leaking then you may be burning the coolant and not know it.
I havn't had too much experiance with the 6.0, the 7.3 is my gig. But from what I have seen, the EGR is usually the cause. Being that you have a tuner, could also point to the head gasket, But that usually does alot of puking at hi power.