Auxiliary light on monitor panel ( black 2, gray 3)

What does that relate to? We don’t have a washer/dryer hooked up. Sometimes it reads 3/4 full other times low/1/3?
Thanks. 2018 315RLTS

Do you have a rear bathroom? If so, it’s likely to that gray/black tank. Gray/black is same tank that handles the sink and toilet together.