Electrical issues 05 vnm

A few days ago I was driving and my display told me my turn signal were out. I figured it was just one but it was actually both that were out. so I stop to fuel up and I shut the truck off. I come back and it will not start, starter engages but wont turn over. I took battery box cover off and cables were dirty. I unhooked cables and cleaned as best as I could and it started up and everything was fine. 3 days later my truck started acting up worse. Now my dash lights are going on/off radio goes on/off, Check climate control module comes on and off. My voltage gauge reads anywhere from 13.4 volts down to 11.6 back up to 12.6. its goes up and down. tested at alt and batteries while running and it 13.8 on both while my voltage gauge said 12.4. cleaned battery cables thoroughly a 2nd time with no change. truck starts and runs fine. im assuming its a ground problem but was wondering if anyone else had a similar issue. Could this also be 1 bad battery in the group? Any helps would be greatly appreciated

 

oh turn signal quit working then work then stop working again. forgot to mention that in the opening part of my post