I do monthly, sometimes several months at a time for work using my 5th for my lodging. I have always left the gray open, except when black is ready to dump, i will close the gray to build up flush water. With the sink and shower having traps, there has never been an issue in the last 25 yrs or so. Dump routine: close gray, fill to approx 1/2. Open black till complete, open gray to start flush while closing black valve (helps clear valve). Open galley gray, and when all gray is complete, close galley gray, leave other gray open till next time.
|