[Blue Bowtie]

You seem to have covered the bases - It's not leaking externally. Is this a dual-heat truck? If so, start looking at the floor and drain openings in the passenger footwell and the rear cargo area.

Just to be sure, check the transmission oil level and condition. You probably see where that is going. If that is all correct, and the engine oil is also not contaminated or overfilled, that only leaves the exhaust system as the exit path for all that coolant.

If it isn't leaking in the vehicle, that only leaves internal system leaks. As you indicated, those are susually evidenced by vapor/white smoke - Usually. A diesel can be a little different.