Just fill the tank about 1/4 full and go for a good dive. The sloshing of the water will clean the sensors better than anything else you can do. Dump the water immediately when returning home, and you'll be good to go for a while again.
But, you can never depend on those sensors to actually work right. It's better to just get more familiar with your camper, learn to estimate your water usage, and fill the fresh water tank often, and dump the black-grey tanks often until you get a good grip on your usage. Then, you'll never need those sensors again.