Abstract: On a recreational vehicle, a water supply coupling is provided to permit the hookup of an external water supply to the plumbing of the recreational vehicle. The coupling selectively permits one-way flow into, but not out from, the plumbing, thereby maintaining the pressurized integrity of the plumbing system. The coupling has a movable check member which may be seated against an annulus to create a seal. In various disclosed embodiments, the check member may be a check ball, a mushroom-shaped check member having a hemispherical seating portion and a stem, and a two-piece check member.