I dump the black tank, close the valve, hook up the flush and then drop the galley tank and let it run a few minutes extra while the galley drips, then close the galley valve. I'll redump the black tank, letting the flush water out then let it keep flushing with the black valve open. Turn off the flush, drop the gray tank, while the gray drains I'll go around and drop in my black treatment. By the time I get back around the gray is done and is ready for a final hose rinse out.
Seems to be the most time efficient method. If I have time at the station I'll just let the flush run with the black open like you're supposed to, but I don't want to hold anyone else up.
Now-2014 Sierra 346RETS 5er BUB
Then-2002 Keystone Springdale 286RLDS TT
Nights camped in 2014-28, 2015-127, 2016-10