OP may be talking about
this support, sometimes called a "slinky."
I use it when I am parked and hooked up for over a week, otherwise I just lift the hose by hand to drain it after dumping.
When the slinky is in place I leave the gray tanks open, black tank closed. When black tank gets to 1/2 full I close the gray tanks so they fill as well and dump them all when the black tank is full. Dump the black tank first, let it drain, then use the gray tanks to flush out the hose.
One other tip, our kitchen gray tank is small and fills quickly. When we are traveling and only hooking up every few days to dump, we use a dish pan for washing dishes and dump it in the toilet to balance out the tanks.