00 Mack CH612 power issue, no crank/start

I just purchased a 2000 Mack CH612 with an E7 engine and Allison automatic transmission, approximately 500k miles, and it will not start. I don't know where to begin.

1. Batteries are fully charged, they were dead when I got them, 6 months of age, interstate group 31, total of 3.
2. I have power to the lights with key off.
3. When I turn the ignition key on, I get engine shut down, ABS, park brake, low air, etc. The engine shut down light does shut off.
4. The shift tower that holds the Allison key pad, it doesn't come on most of the time. Some times, it will come on with the key on, then a click occurs and it shuts off. If I try to start the truck with shift pad on, a click is heard from the indoor fuse center and nothing happens, power to the shifter goes off, nothing is displayed.
5. The only other weird thing I noticed was the tail lights appear to stay on. I've swapped relays, and nothing.
6. The ignition is worn out, I can pull the key out and even turn the ignition with the key 1/4 of the way inside. I can take the key out with the ignition on.
7. Every time I go to start, there is a click from the relay center inside. One time, something changed and it sounded like I was hearing a click from either the starter or a relay under the hood.

This is just a single axle, day cab tractor. If anyone has any ideas, please point me in the right direction. Thanks!

 

How would I diagnose if the ignition switch is bad or if the tumbler is bad and not engaging something in the ignition switch? Thanks.

 

UPDATE: Thank you for the responses. The issue was a bad connection and had nothing to do with the ignition switch, everything works as it should now. Thanks!