Large RVs can have multiple holding tanks that seem to serve the same purpose. The most common situation is when an RV has a gray water tank and a galley tank. Both of these tanks collect and store gray water, but one is called the galley tank because it services the kitchen. And we have also seen that newer large RVs can have two black water holding tanks, one for  the bathroom in the front and for the bathroom(s) at the rear. 

The purpose of having multiples of the same type of holding tank is to lengthen the time required between dumps. Couldn’t this be accomplished by installing larger tanks instead of multiples? Not really, because the  framework required for strength in the RV chassis typically does have room for a single massive holding tank. The plumbing also comes into account when a manufacturer decides to install two black tanks, because they want to make sure that the plumbing has the shortest run with as few turns as possible from the toilet to the holding tank.

An example of an RV with an offset drain from the toilet to the black water holding tank on a 5th wheel RV
An example of an RV with an offset drain from the toilet to the black water holding tank

The longer the run and/or the more turns in the plumbing, the higher the likelihood of clogs. Weight distribution is also a consideration; most tanks have a capacity of somewhere between 25 to 55 gallons, meaning they will weigh 208 to 459 pounds when full. Placing the holding tanks as low as possible and spread out as evenly as possible across the framework of the RV will enhance the RVs handling and stability while driving as well as the RVs fuel consumption (miles per gallon). NOTE: most RV manufacturers do not recommend traveling with full holding tanks because the weight may be greater than tires will allow when the RV is fully loaded, so whether you have “duplicate” holding tanks or not, keep this in mind. 

Now that we understand the purpose of having multiple tanks that serve the same purpose, it’s important to know how to use them and care for them. I started researching this topic by reading through the plumbing section RV owners manuals, but there were no special instructions for use and care of these “duplicate” holding tanks.

A discharge manifold with a grey water and galley holding tank on an RV
A discharge manifold with a grey water and galley holding tank

Next, I called several of the largest RV manufacturers in the US and asked their technical support staff if there were any special instructions for caring for an RV with multiple gray and black water holding tanks. I was told consistently  that there are no special instructions and that the holding tanks should be treated the same as if there was only one of that type of tank. However, even with no special instructions from the manufacturers, there are still some logical approaches to using and caring for an RV with multiple tanks of the same type. 

To begin with, it is important how you will be camping in order to address the use and care of multiple holding tanks. If you are planning on camping at a full hookups campsite, then the manufacturer’s instruction to treat the tanks as if there was just one is pretty good advice.

An RV with both of the gray water holding tanks in the open position while camping at a full hook ups campground
An RV with both of the gray water holding tanks in the open position while camping at a full hook ups campground

For the gray and galley tanks, the knife valve(s) can be left in the open position, with a P-trap bend with water present in the sewer discharge hose. The hose will need to be run to the sewer port at the campsite so that anytime a fixture (other than the toilets) is used, the water, soap and debris can flow freely from the RV into the campground sewer system. The black water holding tank knife valves should be always left in the closed position except during dumping. You should also add a high-quality holding tank treatment to each of the black water holding tanks.

An RV toilet bowl filled with water in between uses to create a vapor barrier from the black water holding tank
An RV toilet bowl filled with water in between uses to create a vapor barrier from the black water holding tank

Since you are staying at a full hookups campsite and water usage is less of an issue, be sure to keep water in the bowl of each toilet in between uses to act as a vapor barrier to keep odors away. And finally flush the toilets with plenty of water each time the toilet is used to allow plenty of water to cover the waste and paper in the black water holding tank. This is essentially how anyone  at a full hookups campsite should use and care for their holding tanks, regardless of how many  tanks they have.

On the other hand, if you are planning on going boondocking in your RV, then you might need to make some adjustments in the use and care of your holding tanks. As with all things in the RV world, your circumstances will affect what you should do. For example, if you are going camping with your family of four over a 3-day weekend, and your campsite has no hookups whatsoever, you won’t need to make any changes in the use and care of the holding tanks. However, if you are looking to stay longer (let’s say 7 days) with the same number of people and no hookups, then water conservation and weight loading may come into play. So rather than letting water flow freely into the gray water or galley holding tanks, it would be a good idea to use  a bucket in the shower to catch some of the water for flushing the toilet or filling the toilet bowl in between uses. If you have an RV with two black tanks that service two toilets, it would be a good idea to instruct everyone to begin by using only the forward toilet so the additional water weight will be more centered in the RV. If that black tank happens to fill nearly to the full level, then instruct everyone to begin using the aft toilet.

The main consideration for an RV with multiple black or gray holding tanks is to make sure that the tanks have not been filled to a level that will surpass the weight limit of the tires.

An electronic leveling system on an RV
An electronic leveling system on an RV

Also, making sure the center of gravity of the RV remains as centered as possible (especially if you have a fifth wheel or towed vehicle) will help to keep the vehicle stable while in motion. Both of these are important for having a safe camping trip. 

Another tip for the care and maintenance of multiple black or gray holding tanks is to alternate which black tank you use while you deep clean the other black tank. This will allow ample time for the cleaning product to work uninterrupted. Alternating tanks in this way will also allow for other maintenance on the opposing holding tank, such as cleaning the sensors, while still being able to use one of the bathrooms. 

In many RVs the gray water holding tank and the galley holding tank collect water, soap and debris from separate areas of the RV.

An RV with two gray water holding tanks with valves located at the wet bay, sometimes one of the tanks is labeled "Gray Tank" and sometimes it is labeled "Galley Tank"
An RV with two gray water holding tanks with valves located at the wet bay, sometimes one of the tanks is labeled “Gray Tank” and sometimes it is labeled “Galley Tank”

For example, a galley holding tank may be used to collect water from the kitchen sink and perhaps an onboard dishwasher, whereas the gray water holding tank will collect the water and soap from one or more of the bathroom sinks or showers, and maybe even an onboard laundry washing machine. Because of this, it would be difficult to alternate between the two tanks.

For the most part, an RV with multiple holding tanks serving the same purpose can be treated like an RV with only one holding tank that serves that purpose. Just be cautious about the possibility of overloading the weight limit of the tires and the potential for the center of gravity to shift if one or more of the holding tanks is at capacity. Outside of that, having multiple black or gray  tanks that serve the same purpose can be a very nice luxury to have on your RV. Happy camping!

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