Electrical tech needed....1986 Westy 150,000mi....

During a short trip yeaterday my Oxy AND Battery red lights came on.
Pushing the reset button on the 'black box' the oxy light still on.
Battery checks +12v (Radio Shack cheap tester). Found no voltage output,
between red and black on the rear of the altenator. Do I assume that the
alt. is kuputt?? Or what else is there to check?

Ed Henckel
'VultureVan'