My guess??? Hooked into the black tank flusher. The hose bibs are close enough to each other that a mistake like that can be made. Should be a valve/handle by the water intake, says things like "city/gravity" or "fresh/tank". That's the line you want. Tank flusher bib usually has a cap of some kind to prevent "dumb things".
However, we ALL KNOW........ __it happens.
Thx....for info....ive put a link to the photo of the water system