Like you, my black tank valve was stuck on my 2017 Mini 2108 TT - it took all of my 250+ lbs yanking on the black handle to finally coax it open; which I did thankfully before my trip.
The Black valve seems to be a loooooong way away from the handle, and the connector to the tank valve has quite a bit of play both in and out. The Grey valve is much easier to operate and much closer to the handle.
When I close the Black valve, I have to push in HARD and FAST to get it to fully close - if I don't, there is a mess to clean up when I pull the sewage cap at the next dump.
I finally installed a tank bung and garden hose connector with a cap in the sewage cap so I can open a Garden hose cap to let it drain into the stinky slinky before I pull the 3" cap in case either valve is not fully closed.
These trailer valves seem to be real problem areas.