The only thing I can think of that would cause such a symptom would be the regulator - which is atttached to (or part of) the alternator on smaller vehicles but might be a separate unit on large rigs. Do you know if Freightliner checked the regulator (not just the alternator).
I know on smaller alternators, should the battery become disconnected while the engine is running, the voltage will increase to 60V or so. But this is a temporary condition, as the high voltage will almost immediately fry the diodes in the regulator.
But not knowing big rigs, perhaps there is a protection circuit to keep this from occuring.
I would be wary about driving it. Call your dealer and ask them to make a house call.
But my money is on something battery/engine alternator related.
The only thing that works on a RV is the owner...