We stay in a campground for a month at a time for last ten years. Always leave my grey tank open, black tank closed until almost full. Grey tank had no solids to clog pipes, if you have a clear bend, you will see that, Water isn't really grey, it is almost clear. I might close grey tank a day before dumping the black so I have a good swoosh of fairly clean water to clean the hose.
Ole and Anne Anderson, Highland, Michigan
'02 Adventurer 32V, Ford F-53, ours since 4/08,Hankooks, Konis, SeeLevel, CHF
'84 CJ-7 , 5.3 Chevy, 3" lift, 33's, Detroit Locker, Fiberglas tub, winch, hi-lift