One of the more chronic problems with RVs are the holding tank level monitors. The typical monitors are so cheap they often quit working within a year or two; just long enough to out-last the warranty.
The classic example of the monitor system failing is dumping out the holding tank (especially the black tank), but the monitor system still shows full. The culprit here is typically the probes in the tank have failed.
The typical monitor on a RV is fairly low-end (electronically speaking), and with easily fouling probes, the system won't work properly for long. These monitors simply use well-nuts to measure the conductivity of the water.
Water itself is a good insulator, but becomes conductive once dissolved minerals and other particulate matter are mixed in with the water. The conductivity of the water can vary due to the dissolved materials that are suspended within, but that is not the issue here.
The monitor systems fail because anything from toilet paper to gunk stuck on the inside of the tank can essentially "short", or cause a conductive path between the well-nuts regardless of whether or not the tank is full or empty. The combination of corrosion as well as gunk build-up on the side-walls on the inside is what eventually causes these systems to fail.
The correct solution would be to rip out the monitoring system and use one of the high performance exterior capacitive monitoring systems; however this can be expensive.
The in-between solution then is to use a product called "Horst Miracle Probes". These probes were invented by John VanderHorst (hence the name "Horst") and are incredibly simple in design.
They function by the use of a Teflon sleeve between the probe tip and the tank side. Simply put, water or gunk will not stick to teflon, so the end of the probe tends to stay clean.
The holding tank version of the probes features a "roof" that provides a shield from above so that any toilet paper clinging to the probe will not touch the probe itself. While you may still have to clean them occasionally, they are much more reliable than the cheap Well-Nuts that are OEM from the manufacturer.
Installation is fairly straightforward. If you have the typical Well-Nuts, you simply pop those out, and replace them with the Horst probes. However, other tanks may have spin-on probes. In that case, the only option is to drill a 3/8" hole adjacent to the existing probe and insert the Horst probes.
If you end up having to drill holes in your tank, remember you only have one-shot at it, and make sure you are using the correct size drill bit, and that you match the vertical height of the sensors you are replacing.