Black and grey tanks

The black tank holds waste and toilet paper from the RV toilet, while the grey tank holds used water from sinks and the shower. Both need to be dumped at a designated dump station or sewer hookup, and most RVs also have a separate freshwater tank that is entirely unrelated and holds clean drinking and washing water.

Keeping enough liquid in the black tank between dumps, rather than adding only small amounts of water with each flush, helps solid waste break down and drains more completely when the valve opens. A tank treatment product formulated for the tank's material and season, along with periodic full-tank rinsing, helps prevent buildup and odor.

A frequent mistake is leaving the black tank valve open while connected at a campsite, which lets liquid drain away and leaves solids behind to build up on the tank walls and sensor. Dumping black before grey, in a single full-tank flush, and closing the valve between dumps avoids most tank problems.