I have a reat black tank dump connection for the rear bathroom and sink. I also have a forward common dump port for the main black tank, and the two grey tanks.
I have the clear 'tank flush' attachment, which I think is useless to flush with.
I always prep my rear black tank by running the sink for a bit to fill the tank up. I'll run the toilet for a few min to add more water if needed for the forward black tank.
Driving to the dump site helps loosen up the nasty stuff.
Fast dump if people in line, I will hook up the back and dump the back tank (black). I'll then hold my breath and switch the hose to the forward dump connection. I'll empty the main black tank, then both the grey tanks at the same time.
Slow dump.. Same, but I use the intank flushing system (spinners) until the water is mostly clearish.
Usually its easier to empty fuller tanks as they sweep the gobs of goo out faster.