Oh come on, its thoroughly simple to keep the two systems separated. If you're super concerned, use redundant backflow valves. Remember that in your house the two systems are connected as well. Besides, its the grey water system, not the black water (which means that technically its more sanitary than what's in your house), and in a closed RV system you should be sanitizing your fresh water tank every now and again anyway.
Surprised people on a DIY forum are having trouble comprehending this simple principle.
