Black water toilet plugged

You can buy a drain valve fitting that has a hose fitting on it to back flush into your tank just before where you install the drain hose. You can also buy a gate valve that goes on the end of your drain line. If you need to back flush up to your gate valve into the tank. I would buy the two adapters I talked about. Put on the back flush adapter on the end of the trailer drain line. Put on the gate valve put the drain hose to the sewer. Close the added on gate valve back flush into the tank with the water hose adapter turn off the back flush water, drain continue until clean water comes out of the black tank.