[Mikendebbie]

The 3931FB (similar to my 3921FB) has two black tanks and two gray tanks.
I leave both gray tank valves open while we are camped. Some folks close the gray valves a day before leaving with the intent of using the gray tank contents as a final flush for the hose after the black tanks are emptied. I never can remember to close the gray valves the night before leaving, plus I just do not want to "hold" the contents of a full bath tub in the tank...probably just a mental thing for me (bad experience years ago, forgot to drain the gray, it overfilled, long story)- but I just don't want to fill my gray tanks. We never dry camp.

There are two hose connections in the very bottom of the convenience center. The one on the left is black tank flush for the front (full bath) black tank, and the other one is flush for the rear tank serving the half bath room. I hook up hoses to both flush connections. I open both valves and let the contents run out until it is a slow drip, then close both black valves and turn on the water hoses. On my unit - the front black tank (half bath) drains and flushes like a champion! Good gravity flow! I will close the valve and let it fill for a minute or so then open the valve and let it drain. Close to fill and open to drain again. I have a clear plastic elbow coming off the RV drain so I watch the stuff coming out until it runs "clear". I will alternate between the front tank and the rear tank opening the valve on one while the other tank fills.

My rear black tank does not gravity-drain as nice as the front tank. The half bath tank empties with a GUSH...the other tank - not so much, decent flow but never a gush! It's been this way since we got the unit in January 2018. I suppose it has to do with the routing of the sewer pipe. I have to spend more time flushing and monitoring the front black tank. Eventually it runs "clear".