FW28Z, here the supposed answer for RV tanks
I doubt that it would be quite as reliable as the external capacitance type, but these are supposed to be the answer to replace the screw probes in an rv tank
Horst Miracle Probes for Wastewater Tanks in Recreational Vehicles RVs
I have read quite a bit about them on other forums, but have no 1st hand experience..... yet.
Then there is the "gold standard"
SeeLeveL Gauges by Garnet Instruments Ltd. - RV Applications