Multiple warning messages, guages read zero, intermittant elec problem

[Motion Clone]

New to me 2015 GMC 3500 dually LML. Very intermittently, Key on running or not, turn lights on or they come on automatically, I get multiple service warnings. Service Anti Theft, Service ABS, Service Stabilitrack, Gauges read zero, radio goes black. Turn lights off and problem goes away. Sometimes problem goes away on its own after a few seconds, sometimes it appears to cycle a few times. But always stops when lights get shut off and never happens when lights are off. A couple times i have been able to pull codes P262B and U0140.

I recently purchased the truck, drove 200 miles home without any issues. Then I pulled it into my shop to do brakes and knock rust off the frame and various other odds and ends to get ready to put into service. After 2 weeks, I started it up (which it starts and runs fine) and was shortly after this problem popped up.

Heres what Ive done to troubleshoot: Checked Batteries while disconnected, 12.6V and 12.5V. Cleaned terminals. Cleaned 5 frame rail grounds, checked and tightened 2 grounds at base of A pillars. Pulled fuse box looking for corrosion, found none. Checked all BCM fuses in all 3 fuse boxes for corrosion or blown, all good. Cleaned the distribution block fusible link plate on top of P side battery. One of the large spade connectors had corrosion, cleaned to bright metal. Pulled BCM to check wires and connectors, all looked good. Pulled the distribution block at rear to rear taillights and connectors all looked good.. Pulled Trailer brake module connector found corroded pin so i left it unplugged. (Had a Service trailer brake warning since i got it)

I had it so it was very infrequently doing this. I then disconnected both batteries and charged them for a couple hours. I reinstalled the cables and now its doing the same problem above only more constantly and without even turning the lights on. So i guess i made it worse.

 

Any suggestions?