Black tank flush

A black tank flush is a built-in internal spray system for the black holding tank: a separate inlet feeds pressurized water to sprinklers inside the tank, which break up solid deposits. Standard on most US RVs from the 2010s; rare in European motorhomes.

It uses a dedicated "black flush" inlet (usually white-marked and labelled "non-potable"). Run for 5–10 minutes after every dump, with valves open. Without regular flushing, the tank develops a solid "pyramid" of waste below the drain that blocks level sensors and clogs the outlet. On rigs without a built-in flush, washing soda plus ice cubes works as a manual abrasive rinse.