Someone mentioned putting a Water Thief onto the non-potable water hose (which usually has no fitting at all on the end of the hose -- it's just bare rubber). One end of the Water Thief is a slip fitting, which goes over the bare rubber end, and the other end is threaded (and can thus be screwed into your black water rinse port).
A Water Thief is a great idea. But be careful when turning on the faucet for the rinse water. If you turn it on too fast and too hard, the back-pressure will blow the Water Thief off of the bare rubber end of the rinse hose, which will whip around like a wounded rattlesnake and will spray everyone with refreshing non-potable water.
Please do not ask how I learned this valuable lesson.