Everything you need to know about the tanks in an RV.

FRESH WATER TANK

The fresh water tank holds the fresh water that comes out of your sink and shower.  Most RVs have a by-pass that allows you to either use the tank, or you can use water from a hose connected to city water.

GREY TANK

The Grey Tank holds the water run-off from sinks and showers.  Generally "soapy" water and that is why it is called the grey tank. 

BLACK TANK

The black tank collects the waste from the toilet. It’s located under the RV, usually positioned next to the grey tank. The black tank should never have anything in it except the drop-ins we provide and RV-safe toilet paper.  Nothing else.  The function of the tank is to hold the waste until you hook up the sewer hose.  After dumping the tank, be sure to add a gallon or two of water to the tank. You want to maintain a swishy environment so that the solids don’t build a mountain, cling to the walls of the tank, or harden over the tank sensor.  The provided drop-ins degrade the material and help to prevent odors from seeping up through the toilet valve.

When you settle at a campground, you don’t want to leave your black tank valve open for a few reasons: it could cause buildup in the tank (which could cause sewer odors), creatures could creep up into your tank, and the tank needs enough content to drain properly.

HOW TO USE THE TOILET IN THE RV

You want to put a little bit of water in the bowl before use to help with flushing.  Typically, you will find either a lever on the floor (to push with your foot), on the side of the toilet, or on the wall next to the toilet. 

WATER PUMP

The RV has a water pump on it to help the water get from the fresh water tank to the other places in the RV (sinks, toilet, shower, etc.).  If you are connected to a water source at a campground, you don’t need to have your pump on.  

DUMPING THE TANKS

Create a plan, pay attention, wear gloves, and don’t get distracted. Gross things happen when we’re not focused on what we’re doing. Have you ever seen the black tank fiasco scene from the movie RV with Robin Williams?  Just kidding, that almost never happens.  

Attach the sewer hose securely to the tank valve after removing the valve cover. Then place the hose into the sewer hole. Some may have a rock for you to place on top of your connector. This is advisable as you don’t want that line getting dislodged. Double check secure hose attachments. Open the valve for the black tank to drain.  Once the black tank has drained all the way, keep the valve open and open the Grey Water valve.  This will help to clean out your hose.  Once the grey water has finished draining, close the black valve and then close the grey valve.

Source: https://thedyrt.com/magazine/lifestyle/black-water-tank-rv/