2007 5th wheel Chapparral 267RLS discharge confusion

Hook up sewer hose to that 'common' 3" line

Open black tank 3" drain valve
When black tank is empty-----------use flush connection on side of rig to flush black tank.
Close drain valve

Open grey tank 1 1/2" drain valve
Drain grey tank
Close drain valve

Add some water back into black tank......2 full toilet bowls flush is good. Keeps bottom of tank wet