I drain black first, grey second, then usually refill both thanks with fresh water and repeat.
The "dump hose" is pretty clean after that, and it only gets used for that, drain fully and screw the ends together and store out of sunlight..
After a few trips to some dump stations where it looks like someone just got close to the drain and opened the valves and let $#!T fly, I decided to find a better way.
Before leaving camp, if we have any ice cubes left, I dump them into the toilet just before leaving, they slosh around and help to knock off any crud sticking to the tank walls etc.
