We've been lucky in that regards. Our Black water tank has a whirly bird in it and I also have one of the attachments where you connect your hose and back flush the tank. I have to say I am always surprised how much stuff comes out. The attachment I mention is clear 45* bend you connect to your discharge connector. Our process is:
1) Connect all the plumbing
2) Dump Black water
3) Use Whirly Bird
4) Dump 5 gallons down the toilet
5) Repeat Whirly Bird
6) Connect hose to back flush on adapter and spray until clear
7) Close off everything and dump 5 gallons of water in Black water tank and add chemicals...
