Battery icon on dash popped up, replaced the battery, no fix. Road testing and the dash and radio display completely powered off, blinkers stopped working. This issue is intermittent/flickers, the dash kicks back on then goes out again.

Took to mechanic this morning and battery and alternator are good, during testing.

How do I troubleshoot this issue? Mechanic said to keep driving till it dies. I tow a trailer and need lights working, that's not an option.