Water reaches an RV one of two ways: from a campground tap through the city water inlet, or from the onboard fresh tank through a 12 V pump. Everything you use drains into two holding tanks, gray for the sinks and shower, black for the toilet, and you empty both through a sewer hose.
The RV Water System Explained: Fresh, Gray and Black Tanks, the Pump, the Regulator and the Hookups
On this page (5)
The water path, start to finish
Every RV plumbing system follows the same order, whatever the brand or floorplan. Trace it once and the rest of the system stops being a mystery.
- The campground tap, or a jug and funnel if the site has no water.
- A pressure regulator on the hose, so campground pressure cannot exceed what your plumbing is rated for.
- A filter on the hose, which catches sediment and taste before anything enters the RV.
- Either the city water inlet, which feeds the taps directly, or the fresh tank fill, which stores water for later.
- The 12 V water pump, which pressurizes the system when you are running off the fresh tank.
- The taps, shower and toilet, plus the water heater on its way to the hot side.
- The gray tank under the sinks and shower; the black tank under the toilet.
- The sewer hose from the RV's drain outlet to the campground sewer inlet or a dump station.
Fresh, gray and black: what each tank does
Three tanks, three different jobs. Keystone notes that on some floorplans part of the gray water drains into the black tank, and some smaller Airstream trailers use one combined gray and black tank.
| Tank | What is in it | How it empties |
|---|---|---|
| Fresh | Drinking and washing water you carry with you | Used up through the pump, or drained at a low-point drain |
| Gray | Drain water from the sinks and shower | Gray valve, through the sewer hose |
| Black | Toilet waste and flush water | Black valve, through the sewer hose |
Tank sizes and valve locations differ by model, so check the holding tank section of your own owner's manual for which valve is which on your RV.
Tip: Dump the black tank first, then the gray. The gray water runs through the same hose afterward and rinses it out, which is why the two valves are pulled in that order rather than together.
Hooked up or on your own tank
On a site with a water hookup, the hose does the work: pressure from the tap pushes water through the inlet to the taps, and the tank is just storage you do not need that week.
Off hookups, you fill the fresh tank first and switch the pump on. The pump senses pressure dropping when you open a tap, runs until the tap closes, then stops. If it cycles constantly, runs dry or will not build pressure, that is covered on the page for RV water pump problems.
Tip: The pump only draws from the fresh tank. It does nothing on city water, so leave it switched off while you are hooked up and you will not hear it chatter every time someone opens a tap.
Keep the hoses apart
An RV carries three hoses that are kept apart and never swapped: the drinking water hose, the sewer hose, and the separate hose used to flush the black tank. Jayco's manual says the drinking water hose is for fresh water only.
- One non-toxic drinking water hose, used only for drinking water and filling the fresh tank.
- The sewer hose, stored in its own bumper tube or bin, handled with gloves.
- A third hose for tank flushing and rinsing, kept with the sewer gear and never used on the fresh side.
Which page covers your part or problem
Water coming in
- RV water pressure regulators: why one goes on the hose and what to look for in a regulator.
- RV water filters: inline, canister and under-sink filters, and what each one removes.
- The fresh water tank: filling it, draining it and sanitizing it.
The toilet
- RV toilets explained: gravity, macerating and cassette designs and how they differ.
- RV toilet problems: a seal that will not hold water, a pedal that sticks, odors.
- Replacing an RV toilet: the flange, the bolts and the water line, step by step.
- RV toilet paper: what breaks down in a black tank and what does not.
- Composting toilets: how they work and what changes if you drop the black tank.
Tanks and dumping
- Black and gray tanks: valves, sensors and how to dump without a mess.
- Tank treatments and cleaning: what treatments do, plus flushing and clearing clogs.
- Sewer hoses and fittings: lengths, elbows, supports and storage.
- Macerator pumps: pumping waste uphill or through a long, small-diameter line.
All guides in this section
- Composting Toilets in an RV: How They Work and What Changes for the Black Tank
How a composting toilet works in an RV, emptying the liquids and solids, smell and venting, what happens to the black tank, and incinerating toilets.
- How to Fill, Sanitize and Clean an RV Fresh Water Tank
How to fill an RV fresh water tank, sanitize the water system with bleach the way builders' manuals say, and drain and clean the tank after storage.
- How to Replace an RV Toilet: What Fits, What to Measure, and the Swap Step by Step
How to replace an RV toilet: measuring the flange and the distance from the wall, what fits (and whether a household toilet can), and the swap in steps.
- RV Black and Gray Tanks: How They Work, Dumping in Order, Valves and Sensors
How RV black and gray tanks work, the order to dump them, slide valves, why tank sensors read wrong, portable totes, and how long waste can stay in.
- RV Black Tank Treatments and Cleaning: What They Do and the Clog They Cannot Fix
What black tank treatments do (enzymes, bacteria, chemicals), how to flush and clean the tank, household cleaners and seals, and the clog none fixes.
- RV Macerator Pumps: When You Need One, Flow, Lift and Distance
What an RV macerator pump does, when you need one instead of a gravity dump, the flow, lift and distance makers print, and the downsides to know.
- RV Sewer Hoses Explained: Sizes, Fittings, Supports and How Long They Last
The standard RV sewer hose size, bayonet and campground fittings, couplers, hose supports, and what the makers say about storing and replacing a hose.
- RV Toilet Paper: What "RV Safe" Means and What Clogs a Black Tank
Whether an RV needs special toilet paper, what "RV safe" and "septic safe" mean, a dissolve test you can run at home, and what else clogs a black tank.
- RV Toilet Problems: Not Holding Water, Leaking, Weak Flush, Clogged
Why an RV toilet will not hold water, leaks at the base or the back, flushes weakly, keeps running or clogs, and the checks in order from the makers.
- RV Toilets Explained: Gravity Flush, Macerating, Cassette and Portable
How an RV toilet works, the main types (gravity flush, macerating, cassette, portable), porcelain vs plastic, and the Dometic and Thetford models by spec.
- RV Water Filters Explained: Inline, Canister and Softeners, Micron Ratings and When to Change Them
What RV water filters remove, inline hose filters vs canisters, micron ratings and NSF standards, softeners, and how often makers say to change them.
- RV Water Pressure Regulators: Why You Need One and What PSI to Set
Why an RV needs a water pressure regulator at campground taps, the PSI makers recommend, fixed vs adjustable regulators, and where to install one.