Learn About RV Holding Tanks: The Basics Of Charging, Dumping, and Flushing

This page contains useful information about your RV holding tanks, including how to maintain and dump your tanks.

The Types Of RV Holding Tanks

Table Of Contents

RV Holding Tanks Repair Man
Learn about RV Holding Tanks with these sections:

First, let's start with the basics. There are three holding tanks in most RV’s:

FRESHWATER TANK This tank holds the freshwater you will take with you when traveling and camping without hookups. It will be used for washing dishes, taking showers, drinking, and flushing the toilet. I personally do not use the stored freshwater for drinking but that is just my preference. I bring bottled water for my family to drink and make coffee. You will need to turn on the water pump to use this water.

I advise new RV owners to travel with the tank at about 1/3 this allows you to flush the toilet and have emergency water should you need it and then your not carrying all the weight of a full tank thus getting better fuel mileage. If your going to be dry camping I suggest you fill the freshwater just before you arrive where you're going at a fuel station or rest area.

GREY TANK This tank will hold everything coming from your sinks and shower. Be careful as this tank is normally the first tank to overfill and will be noticeable in your shower or bathtub. If you have full hookups you will leave the valve coming from the grey tank open and allow the sinks and shower to drain straight through the holding tank and out through your sewer hose.

BLACK TANK This tank is for the toilet waste and requires you to "charge the system" before use to prevent the tank from getting plugged up and not draining. The Valve to this tank always remains closed unless you’re dumping or flushing.


I’m sure everyone has used an out-house at a baseball game or outdoor function and you might have noticed the blue liquid in the tank. Your holding tank needs to be operated the same way. You need to add a few inches of water to the tank and then chemical of your choice to the water that will break down the waste and toilet paper as you use it. I personally prefer Aqua-Kem by Thetford. This is what is used in most airplanes and out-houses and it works good for me. Some RV Parks are on septic systems and require you use chemicals that are safe for the septic system.

WARNING: Do Not use the black tank if it is not charged. Doing so will allow the waste and toilet paper to pyramid and plug up the tank costing hundreds of dollars to repair and in some cases you’ll need to completely replace the tank.

A RULE TO REMEMBER: When you’re hooked up to sewer and you’re camping you should never have both valves open at the same time. The black tank always remains closed because you have charged the tank and the grey remains open to allow the sinks and shower to drain.

When in a campsite with hook-ups, you will have your grey tank valve open and your black tank always closed.

DUMPING Your Tanks

In a campground with hook-ups: When it is time to dump your black tank remember the rule and close the grey valve before opening the black this will keep the waste from draining from the black tank over into the grey.

When the Black tank has drained you’ll need to flush the tank by:

a) Bringing a water hose into the RV and sticking it down the toilet and running water into the tank, close the valve allow the tank to fill to ½ and open the valve. Do this over and over until the tank drains clean.

b) Same as “a” except you can put an adapter on the end of your hose and goes into the tank and sprays the side and you turn it.

c) Some RVs have a flush system outside you can connect a hose and will flush the black tank from the outside. Pay attention to what you’re doing and don’t overfill the tank. Close the valve and allow the tank to fill to ½ and open valve, do this over and over until the tank drains clean.

Regardless if you use A, B or C remember to add two or three inches of water to the tank (you’ve already got the hose hooked up or in the toilet) to charge the system and add chemicals.

After dumping the black tank and flushing the tank. Close the valve so you can now open the Grey tank valve and your camping again.

When traveling always try to have the black tank freshly charged as this will clean the tank and the sensors while you drive and when you store the RV. The dump valves will remain lubricated and the RV is ready for unexpected guests or your next trip.

Dry Camping (Using a Dump Station)

You will need to dump both the grey and the Black tank. When the Sewer hose is hooked up securely to your RV and in the dump station hole:

1) Hold your foot on the sewer hose going into the dump hole or place a rock on it, this will keep the hose from blowing out of the hole and making a huge mess when you open the Black tank valve. When the tank is finished draining use a hose and flush the tank as described when dumping in a campground.

2) If water is not available always dump the black tank first. To flush the sewer hose just close the black valve and open the grey tank valve and this will flush the sewer hose clean for you. Then be sure both valves are securely closed and put the sewer hose away.

3) Go inside the RV and flush the toilet (water pump ON) for a few minutes or more to get the 2 inches of water you need before adding the chemicals. Now you haven’t been able to flush the black tank but after you charge the black tank and drive the RV it will rinse the tank clean.

WARNING: I see no problem not flushing the Black Tank at every dump but be sure to always charge the tank. I would go out of my way the 3rd or 4th time to find a dump station where you can rinse and flush the tank.

TIP: When I’m returning home with my RV and plan to store it for a while I will add a little toilet chemical down each sink to allow the chemical to lubricate the gray tank valve when stored and it doesn’t hurt for a little to be in the P-traps.


Most RV’s have two valves and they are marked Black & Grey

Black is your toilet waste.
Grey is the sink and shower waste.

Very few RV’s have three holding tanks but if yours does that means you have two Grey Tanks and One Black.

Also in some very small RV’s you may only have one tank and its used for the grey and black. Special care is needed when you have one tank.

Be sure your valves open and close smoothly and try to avoid closing the valves when the tanks are being dumped, you do not want the valve to close on toilet paper or waste as this will cause the valve to leak over time.

After charging your black tank you can check the drain to see if you’re dripping on the ground. If you are dripping, check the valve and be sure it’s closed or have it replaced ASAP.


Be sure when you use your toilet you add lots of water, the more the better. The water in the toilet bowl is all you’ll get to flush after usage and the more water you have the better chance all the waste and toilet paper will be sucked out of the toilet and into the tank. This will insure the waste in the tank is under water (not pyramiding) and little chance of the toilet becoming plugged after use.

You do not need to purchase single ply toilet paper but be careful how much you use. You do want to avoid quilted or heavy toilet paper.

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Robbie Warford, San Diego RV Repair

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