Warning is to make sure your black tank valve is open when flushing. If valve is closed and black tank fills, you have a mess.
However, after flushing with valve open, I close it for a couple of minutes, never leaving the area, and then open valve again , it really flushes out the black tank.
I use pressure gauge type fitting for water line. Too much water pressure can damage interior hoses and fittings. Better safe than sorry.