Fresh and Waste Water Systems
RVs have frequently been referred to as a "house on wheels". Just as a residential home will have fresh water and waste water removal systems, so does an RV. Unfortunately we don't have an unlimited water supply and we also can't direct our waste directly into the sewer as we're traveling so the RV's system is somewhat unique. Let's examine the various parts of an RVs plumbing system:
Fresh Water Systems
Any time we run a sink, flush the toilet, or take a shower, we need a supply of fresh water. The fresh water system in an RV is similar to that in a sticks and bricks home in that there are both cold and water lines feeding the various fixtures. There is also a hot water heater that will heat the cold water and feed it to the hot water lines. However, the source of water is vastly different in an RV. When we are connected to a campground that has water hook-ups we can supply a continuous source of fresh water to the RV's plumbing system. But, when traveling down the road we have no water source. For this reason a fresh water tank is provided. This tank can be filled from any water spigot, either at home or when traveling. There is a 12 volt water pump that is used to pump the water from this system into the plumbing lines in order to maintain water pressure. The hose connection will either supply city water pressure to the system or can be used to fill the water tank just by turning the tank fill valve to the desired position. When running on city water the water pump is not required. Because the quality of the water source can sometimes be debatable, a water filter is also incorporated into the system.
An RV's water lines are generally PVC tubing that use the PEX connectors. These connectors can be either the metal band crimp type that require special tools or they can be of a reusable fitting type. My Allegro Bus uses a system of reusable connectors that are very easy to work with and requires no tools. They are made by SeaTech and by selecting the following link you can view the SeaTech catalog - SeaTech Catalog link. It's not a bad idea to keep a few of these fittings in your spare parts, along with a hunk of PEX tubing. Then, if you ever damage a line or let it freeze and crack, you can do a quick repair and be on your way. The PEX tubing and fittings are rated in excess of 100 PSI so it is strong stuff.
Speaking of water pressure, Tiffin Motorhomes does test each coach's plumbing systems to over 100 PSI. I don't know if you necessarily want to run water pressure that high on a regular basis, but the system is tested to that pressure. Some campgrounds have their well pumps set very high so that they can push enough water out to the far ends of their campground. Technically, if the underground lines were large enough they wouldn't have to do that but many times a water system was designed for 50 sites and they have since added more sites and now the plumbing isn't designed for 150 sites so they just crank the pressure up to compensate. It's a good idea to use a pressure regulator on the spigot to protect both your RV and the water hose feeding it. A good tech document that covers this is Mike Sundberg's Water Pressure - You Either Have it or You Don't document found in this section. Be sure to use the diaphragm style regulator with a gauge port, which is commonly available at your home building supply center.
Water filters are another important area to consider. Again, you might want to check out the Choosing the Correct Water Filter article in this section. There are lots of different filters. Some filter well, and some don't. Typically, water filters either have great water flow and poor filtration, or poor water flow and great filtration, but there are a few notable standouts that do a pretty good job at both. You can go with an inline filter or a mounted spin-on cartridge design. The larger cartridge filters cost more and take up some room but they do last longer. In the past I have had good results with the TastePURE inline filter, available from Camping World, which does a pretty good job of filtering water while still passing enough flow. At the present I've gone with a HydroLife HL-200 canister style water filter using a C2063 bacteriostatic cartridge which I get from RV Upgrades that's mounted in the basement water service compartment. You can get it with either silt filters, carbon black filters, or bacteriostatic filters and I've found it to really clean up any water with minimal impact on water flow.
If you are thinking of an Reverse Osmosis system for your RV, I wouldn't advise it. RO systems can't pass enough water at one time to satisfy those needs. They are only useful as an auxiliary drinking water filtration system. If you do want to have the very best filtration, most caravans that travel through Mexico will use the Seagull IV series from General Ecology. These units are not inexpensive but they will filter anything out of your water, including Cryptosporidium or any other bacteria or dangerous "stuff" that might be in the water. We keep one on the cold water side of the kitchen sink to provide perfect water for drinking or cooking. You won't want one of these on your whole system though or else it'll get too costly changing cartridges due to the extra water usage.
Waste Water Systems
What goes in, must come out, right? An RV's waste water is divided into two systems - gray water and black water. All of the sink drains, shower drains, and washer-dryer wastewater is sent to the gray water tank. This is where your soapy water will reside until you dump it. The toilet is connected directly to the black water tank (why do they call it black? Shouldn't it be called the brown water tank?). This is where all the "stuff" from the toilet will go. Each of these tanks has a separate drain valve that goes to a common drain port where you connect your sewer hose to. Your sewer hose just twists onto the dump valve outlet and then goes to the campground's sewer connection or to a dump station.
Whenever you are connected to a sewer connection it is a good idea to keep the dump valves closed. If you leave them open all the time you'll allow solids to build up in the tanks (especially the black tank) and you'll have odor problems. The ideal method is to allow them to fill to at least 2/3rds before dumping. Then, you dump your black tank first. After the black tank is empty, then dump the gray water tank. The gray water will help flush the hose clean. If you are at the same location for a long time you can leave the gray valve open if you are passing a lot of water but you will need to remember to close it a day or two prior to dumping the black tank or else you won't have any flushing water for the hose. One disadvantage to leaving the gray tank valve open is that insects (and even frogs) can climb into your gray tank from in the sewer sao you might find yourself showering with friends.
Many RVs now come with a black tank flushing attachment. This allows you to connect a garden hose to the flushing attachment and run water through a built in spray nozzle to dislodge sediments and help clean the tank out. This is a very handy feature and beats sticking the old spray wand down the toilet (and carrying it back out through the coach when done). When using this, hold off on dumping your gray water until after you have finished flushing the tank. Then dump the gray water to flush all the new sediments from the sewer hose. When you are all done dumping, be sure to add some water to the black tank and add the proper amount of chemical to the tank to allow bacteria to break down and dissolve any future solids. When using a sewer flushing attachment, be sure to use a separate garden hose than the one that you use for fresh water supply. The fresh water hoses are white plastic and have a special liner rated for drinking water. Just use a regular garden hose for the sewer flush to keep them separate so that no dangerous germs can get into your fresh water system and contaminate it.
Submitted by Mark Quasius - 3/05/06