My rig is 5 years old so what I share may not apply today to a 2012. My recollection is poor but I think.....
If you turn on the interior cab lights with the instrument panel dimmer switch, the running board lights will stay on, either for a long time or indefinitely. It won't solve your problem, but at least you can check the running lights for basic opperation. Something to help with troubleshooting.
I had thought something was wrong with our rig's running lights until I realized this. We turned on the interior cab lights with the instument panel dimmer switch, then turned them off individually at the light fixture, much later noting the running board lights were on for no apparent reason.