The Renology shunt should be more advanced . I believe there is a calibration procedure. The going upgrade is Seelevel tank monitor but if what you have works for the tanks not sure I would spend time and money. An accurate voltage gauge can give pretty close reading to lead battery levels.
Is new name of company made Larson tank level system on my 95 Crown Royale it's similar to Seelevel but used two films per tank instead of one like Seelevel. Still works to this day but old school display system ...to an analog VDO gauge and switches.