Plumbing systems on RVs can take many different paths to the various amenities around the RV. How a plumbing system is laid out in an RV depends on several factors. The specific class of an RV can dictate some of the decisions that are made when manufacturing an RV plumbing system. How a system is laid out can change your approach to how you care for the system over the life of the RV. While there are more similarities than differences when it comes to different plumbing systems among classes of RVs, we will focus more on the differences which will change how an RV needs to be cared for. 

RV Toilet and Black Tank Plumbing

Starting at our favorite place in the RV, let’s begin with the bathroom. For most travel trailers, the plumbing from the RV toilet to the black water holding tank is about as direct as possible. Most travel trailer toilets are located directly above the black water holding tank. This allows for the shortest possible distance for the waste, water and paper to travel to the holding tank. Oftentimes all that is required to plumb a toilet into the black tank is an 8-to 12-inch stick of vertical PVC pipe (3-inch PVC)  that is connected directly from the RV toilet to the black tank. When it comes to larger RVs with more amenities, this simple setup can be made more complex because of the vast amount of storage area that a Class A or 5th wheel comes with. Because the storage compartments are located below the floor of the RV, the black water holding tank is typically located at the low point in the RV. This means that the plumbing from the toilet often needs to be longer than if it was directly above the black tank.

Below a 5th wheel RV toilet showing the 45 degree pipe before it connects to the black water holding tank
Below a 5th wheel RV toilet showing the 45 degree pipe before it connects to the black water holding tank

For many Class A and 5th wheel RVs, the 3-inch pipe that extends from the toilet to the black tank may need multiple 45 degree fittings and several feet of pipe to connect it to the holding tank. Because of the length of the pipe and the necessary bends in order to correctly route the plumbing to the tank, there are far more places for clogs to occur. In many of these larger styles of RV, there are multiple bathrooms in the RV. Some of them have multiple blackwater holding tanks but many have just one. For the RVs with a single holding tank with multiple bathrooms, often, one of the toilets will require far longer plumbing paths to terminate in the black tank. The longer the distance the waste, water, and paper need to travel, the greater the risk of clogs becomes. 

Depending on the size and amenities that may have been included on the RV you have, there may also be remote controlled knife valves for opening the black and gray water holding tanks for dumping.

Electronically controlled waste valves in an RV wet bay
Electronically controlled waste valves in an RV wet bay

They may be electronically controlled or manually controlled but with a cable system. In either case, it will be important to make sure that the valves and cables are well lubricated. When the cables and valves are without lubrication, they have a tendency to get caught and can lead to clogs or other problems while camping or dumping. 

Macerators

Macerating toilets are sometimes also installed on larger, high-end RVs.

An inside look at a macerator pump motor showing the chopping blades for an RV macerator pump
An inside look at a macerator pump motor showing the chopping blades

This style of toilet has a built-in motor connected to blades that chop up any waste and paper before it enters the black tank. While they do an excellent job at breaking up the waste, they are susceptible to jams if anything other than waste, water and toilet paper are flushed down the toilet. It is a good idea for any RV toilet to only receive human waste, water, and toilet paper. With macerating toilets it is critical that nothing else is allowed through them to avoid problems. 

Gray Water and Galley Tank Plumbing

Gray water and galley holding tanks can either be combined or separate depending on the size of the RV. Many travel trailers, class B and class C RVs have a single gray water tank that accepts and holds water, soap, and debris from the sinks and showers around the RV. Large 5th wheels and Class A RVs may split general gray water and kitchen water into two separate holding tanks. In these larger classes of RV, they may also have a dishwasher or a washing machine onboard. Both of these appliances typically drain to the gray water or galley holding tanks.

A gray water holding tank valve in the open position at a campground with full RV hookups
A gray water holding tank valve in the open position at a campground

Because both of these appliances require lots of water function and drain that water into a holding tank, it is generally recommended that these appliances are only used at a full hookups campsite with the gray or galley tank valves in the open position as to not overfill the holding tanks. 

Freshwater Plumbing

Freshwater is supplied to the fixtures in an RV either by pressurized water that is supplied by a garden hose to the RV from a hookup at the campsite or by a previously filled freshwater tank and an onboard pump.

An RV connected to city water with a white freshwater specific garden hose
An RV connected to city water

The “city water” supplied by a campground is a nearly unlimited supply of water and does not require the use of the onboard pump to allow flow out of the fixtures.

An RV water tank under the kitchen sink
Water heater with a tank

It is usually heated by either a tank or tankless water heater.

A tankless water heater on a 5th wheel RV viewed from the outside of the RV
A tankless water heater on a 5th wheel RV viewed from the outside of the RV

Tankless water heaters are a newer technology than a standard tank water heater and are more commonly found on 5th wheels and Class A RVs, however because their size is typically smaller than a tank style water heater, they are being found on more and more Class B, C and travel trailers as well. 

When an RV is disconnected from “city water” and relies on the freshwater holding tank and water pump to supply freshwater to the fixtures in the RV, the pump size and pressure will vary between classes. There are generally two pumps that are used in RVs and they are based on flow rate. The most common water pump has a 3 gallon per minute flow rate.

A 3 gallon per minute water pump used in an RV
A 3 gallon per minute water pump

This pump is used in most travel trailers as well as many class B and C RVs but they can also be used in larger 5th wheel and class A RVs. For very large 5th wheels and class A RVs, they can use a 5 gallon per minute pump. Again the size of the RV will help to dictate the size of the pump being used.

PEX Piping

For supplying fresh hot and cold water throughout the RV, PEX pipe is most often used. There are typically two sizes of supply pipe that are used in most RVs on the road.

Under an RV kitchen sink with 1/2" blue and red PEX pipe on the right side for fresh water supply
Under an RV kitchen sink with 1/2″ blue and red PEX pipe on the right side for fresh water supply

½ inch PEX is used more than any other size of pipe for RVs. They are used on every Class of RV from travel trailers to extremely large class A and 5th wheel RVs. The second most common size of PEX pipe used in RVs is ⅜ inch. This is typically used in small travel trailers and some truck bed campers. 

Plumbing For Winter Camping

The layout of the plumbing in different classes of RV can vary dramatically from RV to RV.

An example of a 4 seasons RV camping in winter
An example of a 4 seasons RV camping in winter

Four Season RVs tend to have a greater likelihood of keeping the plumbing in areas that are either heated or insulated in order to keep the pipes from freezing. While this is a good way of protecting the plumbing from freezing, all RVs have limitations as to how cold outside the plumbing can function. For extremely cold outdoor conditions, additional freeze protection may be required. This can include but is not limited to: holding tank heat pads, heated “city water” supply hoses, skirting insulation around the base of the RV and heated insulation around the supply water lines and drain water lines. 

Holding Tank Sizing

The size and class of RV will also determine the size of the holding tanks that are installed on the RV. Freshwater holding tanks can range in size from 20 to 100 gallons of capacity. For Class A and 5th wheel RVs these are typically 60 to 100 gallons. For Class B, C and travel trailers, they tend to be smaller ranging from 20-40 gallons.

A 40 gallon gray water holding tank installed under a travel trailer
A 40 gallon gray water holding tank installed under a travel trailer

The same can be said for the black and gray water holding tanks. These tend to range from 15 gallons to around 100 gallons of capacity with the larger capacity tanks being on the large class A and 5th wheel RVs and the smaller tanks being on the travel trailers and smaller class B and C RVs. 

This has been a very basic introduction to the variety of plumbing setups in different classes of RVs. There can most certainly be differences from RV to RV as to layouts and specific components. We hope this helps you understand what some of the differences may be but every RV is different. If your RV differs from anything described here that is due to the wide array of design choices made by the manufacturers. Thank you.

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