stupid question

As as been noted, light indicators for the tanks are accurate within a 20% range. After that they are useless. When the shower water backs up in the drain, the gray tank is full, when you can see or start to get some whiffs of sewer smell, the black tank is full. Before you hook up to drain the tanks, connect your water to the back flush connection, run the black flush for about 3 min. and continue to fill your black tank as you open your black tank valve. When the black tank is flushing clear liquid, open up your gray tanks. when your grays are empty, close the gray and the black valves. Let the back flush run for 5 min. and then open up your black tank value again and let it drain until only clear water is coming out. Close your black tank valve and let the back flush run for another 3 min. then disconnect your black tank value. You always want some water at the bottom of the black tank. Caution---- never walk away to do another little task or talk to someone when you are back flushing the black tank. You could end up with the black tank over filling out of your toilet:(:(:(:(