I mostly know Leiterwagen or Löschfahrzeug or Einsatzwagen. Feuerwehrauto is what children say. Also “Wagen” makes sense because when there is an emergency you refer to the multitude of vehicles that move out and the group of people as a “Löschzug” an “extinguishing unit” but Zug also means train and a train has “wagons” -> Wagen