1998 Ford E350 Ambulance

[FSFAS]

My Fist Aid Squad has a 1998 Ford E350 Powerstroke Ambulance with what I believe to be the 7.3L engine. The rig has a habit of dying on calls suddenly and the batteries become fully discharged as indicated by the "clicking" that can be heard when you attempt to restart the rig. I figure there is something wrong with the alternator and voltage regulator, but I am no technician. The weird thing is that the gauges inside of the rig indicate that the charging system is working but then suddenly the system appears to stop working. I don't know if that is some sort of controller kicking (since it is an ambulance) or something is wrong.