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RV Education

RV Resources and Tech Advice

Mike Wilson is a RVDA Certified Master RV Tech. with over 25 years experience  and now he's sharing his knowledge with you.
Mike is the president of Wilson RV and developer for The RV Help Desk.

 RV Holding Tank Use

Holding tanks are the built in tanks that hold any waste coming from your RV. Typically, there are a series of tanks. Tanks that hold waste water coming from sinks and showers are termed “gray” tanks. Tanks that hold sewage are termed “black” tanks

There are procedures that should be followed to use the holding tanks properly.
 

Gray tanks simply hold waste water. Any water going down a drain will end up in a gray tank. In some RV’s there are more than 1 gray tank.
Many type “C’ RV’s have tanks mounted under the RV on each side of the chassis frame. Typically there is 1 “black” and 2 “gray” tanks. The easiest way to know if the tank is “gray” or “black” is to look at the waste valves. The waste valves are located near the holding tank exit where the sewer hose is attached. The “gray” valve is the smaller valve (typically 1.5 inches). The “black” valve is the large valve (typically 3 inches). There should be 1 waste valve for each tank.
All the pipes connect together after the waste valves to exit at 1 location.
 

Since “gray” tanks hold all waste water, you need to be careful what you put down the drain. Often times, food or other large items that can go down a drain will end up in the “gray” tank, but may not come out. Any item that sinks in water will go to the bottom of the tank.
If the RV is sitting still for long, the items may not come out when the tanks are dumped. Any items that remain in the tank may cause odor or can interfere with the waste valve operation.

It is important to always have water in the tank before dumping. I recommend not leaving the “gray” waste valve open while camping. A build up of water in the tank will help flush all items out when dumping. Let the water level build before dumping. This is especially important with “black” tanks. Do not leave the “black” waste valve open while camping. Solids will be at the bottom of the tank. Water build up will help remove the solids from the tank when dumped.

Dumping the tanks should be done in the proper sequence. Be certain your waste hose is in good condition. Waste hoses tend to degrade over time. Inspect the hose prior to use. If it has cracks, tears, or holes, replace it. The waste tank exit is always on the RV’s left side (driver’s side).
To dump, you will need to position the RV close to the in ground dump station pipe. The waste exit for the RV will be low. It may be in a compartment or under the side below a compartment. The exit will be a black plastic (ABS) pipe. It should have a termination cap on it.
I recommend latex gloves on prior to dumping. The cap will twist counter clockwise for removal. The waste hose will have an attaching ring on one end. Insert the end without the attaching ring in the dump tube in the ground first. Be certain to hold the hose since some dump pipes go straight down and the hose may be lost if not held. Next hold the waste hose end with the attaching ring under the termination cap while removing the cap carefully. This allows the hose to catch any residual water from the previous dump. Holding the hose under the cap while removing is also helpful in the event of a leaking waste valve.
Occasionally waste valves get objects caught in them, or simply fail and leak. Holding the waste hose under the cap during removal will help avoid a messy spill.

Next attach the waste hose to the RV, turning the attaching ring clockwise until locked on. Once the hose is attached to the RV and inserted in the dump tube, it is time to dump. Proper sequence is to dump the sewage first. Open only the “black” tank waste valve. Allow a few minutes for the tank to empty. Go inside the RV and flush the toilet. Allow some water to run down the toilet to help flush the tank.
If time permits, I recommend filling the toilet bowl with water then flushing several times. Next, with the sewage drain valve still open, drain the “gray” water tanks. The “gray” water exiting will help wash out the “black” tank and plumbing. When the tanks finish (you can tell by sound) close all waste valves. There will be some plumbing with water dripping out after the waste valves. Allow some time if possible for all water to finish running out before removing the hose.
Next detach the drain hose from the RV. Wash the hose with fresh water while still inserted in the dump tube. Store the hose and attach the termination cap.

RV Help Desk Tips for holding tank use:  

Occasionally wash the black tank with a toilet wand after dumping.
This is done by inserting an RV toilet wand attached to a water hose down through the toilet. Be certain the wand is below the toilet exit and in the holding tank. Wear a dust mask and latex gloves (there may be splash back).
Turn the water on. A high pressure water jet will wash the tank. I wash the black tanks on every rental unit we own, every time they are returned. This keeps the tanks clean and odor free.  

Another tank cleaning method is to fill the “black” tank with water, then drive the RV several miles before dumping. The movement of water while driving may help clean the tank.