It sounds to me like you have a bad monitor in your black tank which is a pretty common thing. Since your trailer is "new" I am assuming it is under warranty. I would take it back to the dealer. As far as closing your gray tank so it can fill up, that won't help your black tank issue at all. They are separate tanks. I don't understand what you are saying here. It is not a bad practice to leave the gray tank open when you are camped. You should never leave the black tank open though. Let it fill, flush it out and you should be good to go. If the tank is reading "2/3 full" even after you empty it, you have a problem with the monitor.