Have been in the same club for years, but actually not so much. The grey are normally pretty accurate, the black never reads below 1/3 but above that is pretty accurate. We clean the grey tank (chemical through drain) about once a year. When the black start acting up I throw ice cubes in the toilet just before we pull out, by that evening travel I flush and then it is accurate back to the 1/3 reading. This has worked on all my rigs dating back to the 70's. I use the gages for when to dump.
JimF, CWO, US Army Ret, 100% DAV
2011 Georgetown 378TS
2008 Dodge Caliber Toad