That is usually the black tank only. Make sure you dump the tank again after flushing, if you have closed the valve. There really is no need to flush the gray, as the soap for dishes and showers keeps it pretty clean. Just let them get close to full before dumping. We make sure the day or two before we leave we start filling the tanks for the final dump.
(may have to dump them half full earlier, but I want a good final dump before we hitch and come home).
2012 Ford F250 ext. Cab 4x4
2002 Jeep Wrangler Sahara
2008 Yamaha V-Star 650 Classic
2008 Work and Play 18LT