Black water tank will not drain

We had the same issue with a brand new RV we rented in British Columbia. Tank showed full (on the gauges), and when empty valve was opened a little bit of liquid came off. I was so frustrated i looked with a flashlight on the dumping end with the black valve opened (now seems a very stupid thing to do):E. This is how i noticed the cutout disk blocking the orifice. I closed the valve, added more water, traveled to the next campground where i was lucky enough after dumping the black tank to find the cutout disc stuck perpendicular to orifice. It was very easy to be pulled out with a gloved hand.
I hope this will not happen to my new TT as it would be impossible to remove a cutout disc due to the length of drain i have.