I'm using Fireboy Xintex level gages, for fresh water and holding tanks.

Would not choose that type again. As I wrote above, the sensor uses air pressure in a tube to measure tank level. That's good because it doesn't clog, and it works fine at-rest, but the system is slow to react to changes and doesn't give a reliable reading while the contents are sloshing underway. Fortunately my fuel gage (which is more critical) uses a traditional VDO level float and is more accurate, even underway.