A few weeks ago replaced a dead aux battery. Week later, no start, battery completely dead.

So I have charged it up.
At inside lower cig lighter, I inserted a volt meter
Battery has been showing 12.3-12.7 when key is in on position. Seems to drop a few tenths over night.
When switched to ready, jumps to 14.2

For last week have had no issue.
However, on a ride yesterday, I switched the video display to the trip info where it shows the pictures and arrows of what is working/charging, etc
I noticed the display for battery was down to about one bar (Usually its near the top) I was traveling on a semifast road 40ish mph.
When I turned into my neighborhood, and started driving a little slower, I noticed the bard starting to rise, and within a few ins was almost to the top.

I did a parasitic drain test on all the fuses last week. Found nothing strange, checking the voltage drops across them.

Today I am going to check the neg battery cable ground, and go have the battery tested. I know sometimes you get bad ones. autozone

Any ideas here of whats going on?
Should the battery be charging up to more in the 13+ Volts?