Since RV is used difficult to tell how long those monitors haven't registered properly. Some RVers don't worry about them & some make sure tanks are empty every time. On the black tank some RVs have flush systems installed. If not you can make up a easy flush system or buy them from Walmart that go into commode with hose connected to flush tank. I made my own with 30" of 1/2 copper pipe. Valve & hose connector on one end. Cap on other end. Then drill two 1/4 holes into side of cap opposite each other. Works great. For grey I use a grey water treatment or some strong soap through kitchen drain. It may take a while to get them clean again. I make sure the monitor reads empty every time. It does take a little extra effort. Some thoughts. Keep in mind. The less grease & soap the less problems in the grey tank. JMHO.