[SteveC7010]

RV fresh water plumbing systems have been notorious for this annoying behavior for years and years. The pressure sensor in the pump only needs a drop of a few PSI to trigger the switch and engage the pump. The pump runs for a second or two, reaches the correct pressure, shuts off and the cycle starts again.

All it takes is just a few drops of water per hour to make all this happen. So, the most common causes seem to be the toilet valve or a sink faucet. The toilet valve has an O-ring in it that is easily clogged up with mineral scale or other debris like sand or sediment. Even algae can cause this. Sometimes the seepage is so slow that you don't see it, and if you don't check the entire circumference of the bowl for damp, you'll miss it. Sink faucets can be the same way.

The pressure relief valve on the hot water heater can weep. It is the nature of the beast that mineral scale will build up on the valve. If you test the valve by opening it, do so several times to clean off the scale. Otherwise, it may have a very slow leak.

The check valve on the city water connection can also be easily choked up with sediment, scale, or sand. And, it's an O-ring in there that can easily curl out of its proper place. When we dry camp, I cap the city water connection with a screw-in plug to be sure it doesn't leak.

Someone already pointed out that the check valve in the pump itself can leak, pushing water back to the fresh water tank.

Note that there is a repetitive theme to all this: sand, sediment, or other debris gets in the system and prevents a valve of some type from fully closing. Except for mineral scale buildup on the hot water heater pressure valve, the other debris can be minimized by the use of a sediment filter on your fresh water hose. There are all kinds available at the RV stores, or you can buy a standard size residential one at Home Depot, etc. and just pick up a couple of garden hose adapters in the plumbing aisle. The filter cartridges themselves are available from sediment grade for $5-6 each all the way up to taste, odor, and chemical filters for $30+ each.

Campground water systems are no different from residential ones when it comes to sediment and other debris. They all have some, even our homes. Campgrounds can have more sediment because they are often on proprietary wells and pull a large volume of water from relatively small small wells which tends to bring up more sediment. We don't see problems at home on city water so much because we run the water and it gets pushed out of the way. Those of us with wells know better. In our RV's, we tend to use a lot less water so the debris doesn't get flushed as well and settles where it can cause problems.