2012 Kenworth Won't Turn Over

I have a 2012 Kenworth with the ISX Cummins. Went to start it this morning and couldn't get the engine to turn over. The starter solenoid will click when I turn the key. Got my volt meter out and had 12 volts at the starter. Switched the starter relay with the horn relay and got the same outcome. Beat the starter with a hammer while someone else turned the key. Turned the key 30 times and shook the ignition switch thinking it could be the ignition. Nothing. Yesterday it did this the first time I tried to start it but turned over the second time I tried. It was 0 degrees out so I didn't think anything of it. The only reason I'm not 100% sure the starter is bad is because a couple days ago it didn't do the "self check" thing where it clicks 8 times when you first turn the key on. It did that about twice that day and hasn't done it again since. Makes me wonder if the ignition is bad. I'd hate to go through the headache of pulling the starter off and buying a new one just for that not to be the problem. Thoughts?

 

Just the solenoid. There was no voltage drop at the solenoid when my helper would turn the key to try and start it. Would getting 12 volts to the starter mean the starter is bad, and not getting voltage mean the ignition is bad?