IS it normal for the panel to show full batteries when plugged in? YES
Why: The panel judges and displays state of charge based upon the system voltage, which is being supplied by the converter when you are plugged in, NOT by the batteires. FULL is 12.6 volts, NORMAL converter output in float mode is 13.6 Somewhat "Over full" far as the panel is concerned.
Nothin adds excitment like something that is none of your business
Kenwood TS-2000 housed in a 2005 Damon Intruder 377