2001 DS with Cat . I'll assume Freightliner chassis and the OEM Leece-Neville 160 amp alternator .
Have you had the chassis batteries load tested ??
I just had my L/N bearings replaced and during the bench testing the shop noticed the regulator was defective ( not allowing full output. )
So the alternator mounted regulator is a possibility .
I'll add to Brett's list of connections to check .
The charge wire from the alternator , does not run directly to the chassis batteries , it connects to the battery cable at the starter . So the connection there has to be clean and tight too .
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.