After you have drained the black tank and the grey tank, attach a garden hose (preferably not your potable water hose) to the tank flush fitting. Turn the water on full blast. Let it run for several minutes. If you have a clear section on your drain hose, you can watch to see if your water is running clear. If not, then continue to let the flush run until the water is clear.
The tank flush is simply a spray nozzle mounted inside the tank. Spraying water through it rinses off the various surfaces of the inside of the tank. Some nozzles are more effective than others.
Once you've decided you are finished washing down the tank, simply turn off the close the drain valve, turn off the water, then remove the hose. The reason you close the drain valve first, is to leave a bit of water in the bottom of the tank. This will help prevent any solids that escaped flushing from drying up and getting hard.
2014 Coachmen Catalina 253RKS
2016 Keystone Summerland Mini 1750RD
2012 F250 Lariat 6.2L Crew Cab FX4
Reese dual cam SC/WDH