Fresh water tank

That sounds more like your potable water tank drain line to me. Usually, near the water pump is a diverter valve. In one position, it allows water to pass from the tank, through the pump, and to the fixtures in your camper. In the other position, it allows the tank to drain out onto the ground. It appears your valve is in the "drain" position. Try turning it to the other position and fill your tank.

If the plastic tube comes out the bottom of the RV it is the fresh tank drain that someone left open. Trace the tube back inside and you should find a valve to close somewhere in the line between the tank and the open end of the tube.

Once the tank is full it will run out the overflow. This is perfectly normal and it is how you tell the tank is full.