The cover Tiffin is referring to is actually a panel about the same size as a compartment door. It should be on the passenger side just in front of your rear wheels, and will have your furnace vent outlets mounted on it. Tifffin has used a few different means of attaching this compartment cover, so you'll have to see how yours is attached. Look to see if there is a piano type hinge along the top of the panel. If so you should have two "L" brackets at the bottom of the panel. Unscrew those brackets and the furnace outlet cover, and the panel should swing up. You will need to use something to hold it up as it does not have struts. On the later Phaetons like mine there is no hinge. The panel is held on by four brackets, one at each corner. After unscrewing those you can pull the panel straight off without removing the furnace outlet cover.
Once you have the panel removed you can get to the black tank sprayer. Remove and drill the holes to 3/32". When you get to this point, see if this link helps: http://photobucket.com/black_tank_flush