For voltage to go from "good" (i.e. over 12.8 VDC)to so low that neither the engine nor the electric brake boost would work doesn't sound like an alternator to me.
That assumes you were monitoring voltage.
More likely, there was a connection/fuse issue near the chassis battery. That would be more likely to result in voltage going from OK to doesn't work at all.
More insight on whether this WAS a sudden loss of 12 VDC or a gradual loss until voltage got so low that nothing running on 12 VDC would work would be enlightening.