A Holding Tank Primer

The following is an excerpt of an article that appeared in CyberSam, the Good Sam Club’s e-newsletter.

One of the most common mistakes new RVers make when visiting an RV park with full hook-ups is to connect the sewer hose, and then open the black and gray tanks. Bad idea–at least for the black tank. When you use the toilet, the liquids will drain away down the sewer hose, but solids and tissue will be left behind. In the short term, this creates odiferous build-up, but in the long term it can create deposits that can be difficult to remove, and may even require tank replacement.

Instead, keep the black tank valve closed, and watch the monitor panel like you would when dry camping. When the tank is 1/2-3/4 full, go out and pull the handle—now there’s sufficient force to flush the tank clean. Since the gray tank primarily handles soapy water, you can leave it open without fear of problems.

One of the most frequent letters or e-mails we receive are on the subject of tank monitors—more specifically, how to make them read fluid levels properly. In most of these cases, the root of the problem lay with the design of the monitor itself; many RVs are equipped with probe-type monitor systems. These systems use screws inserted through the tank wall to measure tank levels through electrical continuity. Power is applied to the screws, with a ground probe located at the 1/3 level. As the level rises in the black and/or gray tanks, the monitor will indicate a new level of fullness as each screw is covered with fluid.

Unfortunately, any time one of those lights is illuminated, all it means is that you have continuity, not necessarily a tank that is at the indicated level. Moisture or tank deposits conduct electricity, too—and the older the tank, the more likely you’ll get false readings.

Though this condition is viewed as inevitable over time, you can prolong proper sensor function by thoroughly emptying the tanks. You should wait until your black tank is at least half full before emptying; if it’s not, add more water. If you have a friend with you, have them watch the black tank monitor (if it works properly, if not, they’ll have to look inside) fill the tank with fresh water either through the toilet or through an exterior black tank rinse fitting, and drain again. Repeat the process at least twice to ensure the tank is clean. If you want to be sure, you can use a clear plastic adapter so you can actually see when the tank runs clean. It sounds gross, but it takes away the guesswork.

When you’re sure the black tank is clean, close the valve then drain the gray tank. The mostly soapy water will rinse the valves and sewer pipe, but you should still rinse the sewer hose with fresh water (using the dump station’s hose) when you’re all finished. Many of us may not have had the luxury of owning an RV since it was new, and so may have to deal with odors and/or sensors that don’t function properly due to residue build up. Many RVers have written to us and said that they have had good luck filling the tank with water, a tank treatment or cleaner/ degreaser and ice, then driving around. The ice helps agitate the tank contents, and, theoretically, knocks residue off of the tank walls. We haven’t experimented with this method (we haven’t had to), but it could work.

If the tank is clean and odors still persist, the problem may be traced to a toilet valve that isn’t sealing properly (due to wear or an obstruction of some kind) or a blocked roof vent. Like your home, the sewer system in your RV is vented to the roof, and may become blocked by a bird nest, bee/wasp/mud dauber nest, or some other obstruction. Check it out first before you take the trailer to an RV mechanic.