2015 pete battery drain

cleaned battery cables disconnected inverter did test with test light will not light up between neg cable and post peterbilt did a test with cable volt meter said they couldn't find a drain claim they let truck set over night and it started next morning drove truck home sat 10 hrs batterys drained to 9.5 help please checked alt and starter also ran battery cables nothing touching any where def heater is not on cant figure this one out if I disconnect the first battery truck will not loose voltage anyone have any ideals

 

thanks I will try that I think peterbilt only let the truck sit 6 hrs after they charged the batterys I will try alt agian