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.
2022 Keystone Cougar 24RDS
2023 Ram 6.7L Laramie Mega Cab