The inlet coupling. The pump will typically (some are not) be marked adjacent to the inlet and outlet fittings. I'd consider just blowing the system out with compressed air and using a bit of AF for the p-trap in the kitchen.
K&L + the Wild Bunch TT: 2011 Rockwood 8293RKSS TV: 2019 Dodge 3500 SRW Crew HO CTD
On most if not all Rockwood or Flagstaff trailers you do not have to disconnect anything. On the outside somewhere, usually near the city water connection will be a hose fitting the same as the city connection marked antifreeze or winterizing. Just get a short length of garden hose and connect it there with the other end placed in the jug or bucket of pink.
Then there will be a 3 way valve on the pump suction line you need to turn which isolates the pump suction line from the FW tank. Might be hard to find. Run the pump and suck the pink right into the system.