2015 Peterbilt 579 - Intermittent No Start - No Communication with ECM

My 579 is acting possessed. Sometimes I turn the key and nothing happens—no gauges sweep, no starter click, just a dead dash. Other times it fires right up like nothing is wrong. When it fails, my scan tool says it can't talk to the Engine Control Module. I've checked the batteries and they are sitting at 12.6V. Cables look tight and not much corrosion on the posts. It seems to happen more when it rains or after a wash.