Q? Holding Tanks:  How can I tell which slide valve on my RV is for my black water tank and my gray water tank.  And can I leave both valves open when I have my sewer hose connected to my RV and the RV Park's sewer drain?

A> Most RV's have two slide valves on them and the black water slide valve is always the large of the two.   It is know as a 3" inch slide valve while the gray water tank, (your sink water drains and shower drain) are also the smaller of the two.  They are either 1 1/2" or 2" drain valves.   As for leaving both valves opened, this is a big, big mistake that is done by a lot of new RVer's till they get their black water tank plugged up and are in need of a service call!!   RV toilets are designed to save water, not flush everything through the holding tank and sewer hose to the parks drain.  You should never leave the 3" valve open, but you can leave your gray water valve open while in a RV park connected to the parks sewer drain.

T<> It also helps your black water tank to only use RV & Marine toilet tissue paper that is biodegradable to brake down in water along with holding tank chemicals.  Also another good idea is one or two nights before leaving the RV park that your staying at is to close your gray water valve.  This way on the morning that you are leaving and you open your black water valve to empty your tank, then you can open your gray water valve to flush out your drain hose before unconnected it and stowing it away.  Return To FAQ