Trouble with grey tank dumping using garden hose

Air must enter for water to run out. While the tank valve is open air enters one of the tank vents, under the sink or through the roof. If you don’t let the tank drain completely the water in the pipes won't drain out, it won’t have any vent. My clear elbow has the fitting to hook a hose to for flushing the line, works great for letting air in to drain the lines.