2018 579 Peterbilt Cummins x15 “Won’t start”

So last week when I tried starting my truck it would crank over for a few seconds and stop. Turn the key and try starting it again it wouldn’t crank at all seemed like it had zero power at all, Turn the key back off and on the 3rd time it cranked over and started like normal. Then again on Thursday it did the same exact thing. Today when I started it up for the day it started right up. Got to where I had to load and shut it off and it did the exact same thing as last week. But today when it did it the first turn when It cranked over the crank seemed real week, taking it to the shop today but wondering what you guys/gals think it might be ?