Storage tanks sensors

Those sensors are notoriously inaccurate. You can open the flapper valve and look in the toilet and see if the black tank is full. If you don’t want water to run while you’re doing that disconnect city water and leave your water pump off.

If water comes up the shower drain then the gray tank is full.

Don’t want to wait till the tanks are completely full? Neither do I. I fill the fresh tank (I don’t use the city water connection even if city water is available). Drain the gray and black tanks. Since all 3 tanks are 30 gal it’s impossible to fill the gray and black tanks before the fresh tank is empty. The fresh tank is translucent so you can see how much water is left. When it gets low I drain and refill. Problem solved and no need for the sensors.