OK, here is the first obvious question:
you did pull the galley tank valve to dump the tank, right?? The one that is usually hidden somewhere under a slide, aka, a LONG way from the gray tank valve and/or the black tank valve???
Not sure how this is arranged on the newer units with that fancy all in one command center for water, electric, battery, etc., but that is how it is on our 04.
There are THREE valves, one for each tank.
Sorry, not meant to insult, just the first thing that came to this ole brain!