A drainage system for emptying a holding tank of a vehicle includes upright telescopic members for attachment to the vehicle outlet. Ninety degree elbows are joined in a swiveled manner and are carried by the upright tubular members and are in communication with inclined telescopic tubular members terminating in communication with the inlet of a sanitary waste system. Couplings of the present system include bayonet inlets having multiple O-rings in place thereon for sealing engagement with a bayonet outlet. The upright telescopic tubular members permit positioning of the inclined tubular members at an incline to ensure optimal drainage. Additionally the upright tubular members permit the inclined tubular members to be swung about an upright axis to facilitate attachment to a sanitary system inlet. A modified form of the system includes an elbow equipped with a bayonet inlet with multiple O-rings for coupling with the vehicle horizontal outlet. A chevron seal prevents leakage between the inclined telescopic tubular members. A lock is insertable in a bayonet inlet to prevent rotation of joined inlet and outlet.