[bshgto]

all you need is the valve below everyone should have this on their camper any way it prevents those little surprises when you take the cover cap off your 3" drain pipe. All gate vales will leak at some point and leave a mess for the next camper, sometimes right at the fresh water valve....YUK
What I do, a day before we leave I close the gray tank valve and let the gray tank fill up on moving day last thing I do is dump the black tank until empty, then close your new external vale and open the the gray tank valve this back flushes the black tank with about 30 gallons of gray water then open your new external valve to complete. You would be surprised how much "stuff" is left in the black tank after you do a normal dump routine.
The flusher for the black tank that came with the unit never really did anything worth messing with. I also found if you raise the front of the camper to max height while doing this a lot more comes out as the tank exists are in the back of the tanks. YMMV

https://tweetys.com/camco-39050-read...r-adapter.aspx