Disconnect the batterys.
Clean & inspect ALL the battery cables - to starter, to junction block,even the ones on the back of the shut off switches, then secure them.
Don't forget the ground cables too, even from engine block to chassis.
I would then check the voltage drop on each cable, one at a time.
If you get more than .3volts while cranking, I would replace it.
Is the starter motor "rebuilt", "remanufactured" or "new" ?
It is possible that the starter motor could be defective, you would have to check the current draw on it while cranking.
Good luck with your repair.