First do you have a clear elbow or straight adapter? This is to ensure that you can see the water that is running from the black tank is clear.
Now what we do; is my wife dumps about 5 to 6 buckets of water down the toilet as I flush the tank outside with the black water flush connection on my trailer. This ensures that the tank is clean for the next trip. Also on our clear adapters which are the FLUSH King type. They have a hose connection that allows you to spray up in to the drain pipe this too helps.
2016 34RL CC; 2008 Ram Mega Cab 2500HD, 6.7L, 68RFE 6 speed, 4X4, Smarty S67, 100K+miles