Many RVs have the bathroom sink draining into the black tank. This was true of my full sized 43 foot diesel pusher Tour Master and for many other models as well.
It makes sense when you understand that most users do not add enough fresh water via the toilet for proper operation of the black tank additives to control odor, etc. If you are hooked up to shore water in a park, I find that a week of black water tank use will make a good volume to drain, followed by a full tank of grey water. My grey water was open to the park's hookup most of the time, but on the weekend I shut the gray valve and did a few loads of laundry and after a couple of showers and other gray water use I was ready to drain the week's black water accumulation followed by the full gray water tank being drained. The TM has a built in tank spray accessory, so I always used that feature to fully rinse each tank in turn, then closed the black drain for another week of live-aboard lifestyle.
Never leave the black tank valve open when connected to an RV park hookup -- you need the fast flow of a full drain flush to move solids out of the tank. For deep winter weather, you also will need to close the gray water valve, as incremental flows of even non-particulate water will cause an ice buildup in the hose connection. In freezing weather, drain the gray water as needed, and the black water every other time you drain the gray...
2007 Tour Master T40C
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