This posting is mainly for people new to the fun of owning an RV.
We've been camped here since Jan 14th. It has occurred to me to look at some of the gray/black tank dump valves as I walk around the resort, to see how many leave them both open - several do. This obviously doesn't count the ones who have docking stations.
Leaving the black tank valve opened all the time will allow the sewage to dry on the walls of the tank as well as the sewer line. Leaving it open may be convenient, but you're heading down a slippery slope. The gray tank, will after a while, also develop a sludge. That tank should be closed once in a while till full to prevent this.
Some of us use no "chemicals" in our tanks. We use "probiotics". My LED's indicate properly. Yep, they show EMPTY when they should - nothing sticks to the sensors.