I had a similar problem, when i charged the camper by plugging the solar panels into the camper trailer anderson plug i had that same voltage movement, when I attached the panels directly to the battery no more movement, try a more direct connection to the batteries then measure the voltage directly across the battery (Connecting the panels via their voltage regulator each time)
It will be the Solar regulator cycling when the battery is almost fully charged. An Analogue meter responds to voltage fluctuations instanty, a digital volt meter doesn't & depends on its update times. I still use analoge meters when measuring fluctuating voltages.
