[RMcNeal]

I also have a 2019 3791RD. One thing I do every now and then when dumping/flushing tanks is to run the nose up high and drain the black and shower gray tanks, then run the nose down low and drain the galley gray tank. The way they are installed and plumbed, the black and shower gray tanks empty to the rear and the galley gray empties to the front. This helps get everything out of the tanks.
The twist on Valterra valve mentioned above is a "must have" too.