Agree with Dave on the gray tanks. When we set up, I open both gray tanks and leave them open. I try to remember to close them the day before we are scheduled to leave. If I forget, I close them and turn on the faucets in the kitchen and bathroom and let them fill before breaking camp.
Just something to flush the drain lines after emptying the black tank.
The reason the shower didn't back up is, they are two separate tanks. That is why you have two drain handles.
You just have to learn the tank levels by experience, because the sensors across the board are extremely unreliable. Be sure not to let the black back up, for obvious reasons.
