low voltage

[Bill in pa]

5 ton 1971 M813 a1 250 cummins.
I tried to start the truck today at lunch, started great this a.m.
Turned over very slow. tonight just click, click, click. Had to drift to start to get it home.
battery voltage reads 23.5 + to +........ 11.9 + to- .......
that's with the truck OFF. Volt meter on dash was in the yellow when running.
Any ideas, Batteries look good, date code not marked.
Im thinking alternator..

 

[KsM715]

Get it started and check voltage with it running. If it's low then best bet would be the alt.