The grey-water holding tank stores waste water from sinks, baths, showers and laundry facilities in a motorhome, and should only be emptied at an authorised dump station. The grey-water holding tank is usually slung beneath the rear of the motorhome, oriented so that the evacuation line empties to the driver's side. The removal of a motorhome's grey-water holding tank may be required for service to or replacement of the tank itself, or to gain access to a component or piece of flooring that needs attention and is shrouded by the tank. The procedure is simple and straightforward.
Drain the grey-water holding tank. Most dump stations in campgrounds, rest areas and service stations have hoses for this operation; if one is convenient, flush the tank several times until clean water comes out of the valve.
Move the motorhome to a flat, level surface where all-around access is unobstructed, and passing traffic is not a hazard.
Release the sewer connection from the tank. Typically this is done by unscrewing a large hose clamp that holds a flexible hose to a rim moulded onto the tank, or by unbolting a plate on the side of the tank to which a rigid sewer line is permanently attached.
Release the smaller inflow pipe from the holding tank; look for it toward the top of the tank and to the fore, where the kitchen sinks and bathrooms would vent. Inspect the tank all around to be sure a second inflow pipe does not exist, perhaps from the back or side where the laundry facility would vent.
Check to see if your tank has a vent pipe that allows gasses to escape through a vent above the roof. If one exists, it must also be released from its anchor points.
Consult your owner's manual or contact the customer support team at your motorhome manufacturer's website to learn how the tank is held in place. Some manufacturers use holding tanks that are screwed or bolted to chassis members through flanges on their sides or tops. Others use straps that are bolted to the chassis fore and aft of the tank. Some screw the tanks directly to the underside of the floor.
Follow the instructions for releasing the tank from its fasteners. Wear protective clothing and a long-sleeved shirt, and have an assistant help support the tank as you remove the fasteners or straps. Drop the released tank downward and onto the floor, then slide it out from under the motorhome.
Some manufacturers protect their holding tanks from road hazards by using an underbody panel or panels. If present, these must be removed to gain access to the tank.
There is every likelihood that draining the tank will not exhaust all the fluid inside, and some may still splash out when the tank is released from its anchors and rocks as you lift it down.