Peterbilt with ISX15 starting issues

I’ve looked everywhere with no answer. So I have a 386 Pete with the isx15 Cummins. It starts fairly slow most of the time. Like the battery’s are weak. Then it got to where it would just click but you can feel the starter teeth engage but it’s like something is killing the power to the starter before it starts spinning the engine. So I’ve replaced the batteries and the ones I pulled out were actually new. All the battery cables look new. I replaced the starter. So apparently whoever had the truck was having the same issues and tried to fix it with cables and batteries but didn’t. The day we put a new starter on it it fired up super fast then the next day it was slow again. If I start it everyday and run it then it’s fine. Still kinda slow. But if I leave it over the weekend or for a few days it just clicks. But I can grab two screw drivers and cross out the starter and it will fire right up every time. It’s a 2012 model so it does not have the solenoid on the firewall. Thanks and any info will be appreciated