03 kenworth t800 no power/no start

1st, i assume test light is hitting ground post of the battery and therefore you should expect nothing at the bolt. (if attached to the positive post and the bolt head, obviously this is the desired outcome)

2nd, is this an led test light? If so, while not an optimal outcome, some bleed-through seems to be normal on most older kws, it (imo) rises to a high priority to correct only when using an incandescent tester.

3. Check the positive cable from the battery to the starter and starter to the alternator, could also be unwanted contact in the alternator itself

4. What are the individual batteries fully disconnected reading in volts? A test light wont tell you all that much other than there is or issnt current flow, an led test light is the worst, you could have as little as 1.5 volts and it would happily light up, thats not enough for anything else though, the computer needs at least 6-10 volts to even TRY to do anything

 

With none of what i asked answered, im just going to guess you have a parasitic draw resulting mostly but not all dead batteries, best i can do with no info

 

turn the key to run and the lights on.
use a test light with a bulb in it and you need a jumper wire to extend the length. connect one end to a negative battery stud and search with the other end. negative stud on starter, frame and cab are a good start. when the light lights work back toward the battery and find the open circuit.