I had a portable sani-system with macerator and pump that was used to pump into 30 gallon barrels at a remote golf course (Kinzua Hills) near Fossil, Oregon. We then took the barrels into Fossil for dumping into the city sewer system.
We first dumped the black tank until empty, then the gray tank until empty. When the black and gray tanks run empty, then the pump should run freely due to no load. Before closing the gate valve on the gray tank, very slowly and only slightly crack open the 4 " connector fitting from the RV, then immediately close the gray tank gate valve. Your sani pump should suck air through the 4" fitting connection that is at this point only slightly cracked open. Because it is sucking air, very little to no leakage should occur. Carefully remove and raise the sani 4" connector fitting up so that the gray water runs down to the pump. At this point, from a jug or garden hose, we poured a gallon or two of fresh water into the open end of the connector fitting to clean the fitting and hose to the pump. Disconnect the power to shut off the pump and carefully lift the sani system up, keeping the open 4" connector fitting above the pump, and the pump above the drain hose going to the sewer, or barrel in our case. The drain hose should completely drain by gravity in a few seconds. After our last RV to be dumped, if we had access to a water hose then we would run several gallons through the sani system to rinse it from end-to-end. We used to dump from a few to more than a half dozen RVs in succession this way every week or two.
