You can replace the "Guts" of that parallex with a Progressive Dynamics 4600 series and sticker the door (One of the best smart chargers made.. Most of the rest are also Progressive Dynamics)
13.8 is not alarming at all when it comes to voltage 13.6 is normal float as I recall. 0.2 might be meter error. I'd not necessarily trust the calibration on those meters less I did it (and yes.. I've done it).
Home is where I park it!