On our previous camper (2014 Laredo), we had the same issue. I had the dealer check it out with a few other things at the time and was told the check-valve in the black tank flush line was installed backwards. It was repaired under warranty.
On our current camper (2017 Premier), we had a loose connection under the bathroom sink where the flush line enters the camper. Discovered this on our maiden voyage when water was running out from under the camper.
Since there was no room to work on it under the sink, I had to remove the female hose receptacle from the outside, pull the line out through the hole a little bit, tighten the clamp then re-attach and re-seal from the outside. Worked as it should have from the factory.
When it works properly you can hear water going into the black tank.
Hope this helps!