Last year the holding tank vent line was plugged up from either backed up sewage or a bug nest. Regardless I got it cleared; you can find my article in the archives. During the winter it became plugged it again. Our P42 has the shell shaped chrome scupper, so I used a cone shaped garden hose nozzle attachment to apply city water pressure to break it loose. No such luck! So I pull up the sole and disconnected the vent line from the tank. The line tested clear. Then I checked the bronz elbow that attaches to the tank. Plugged solid! I used a bent wire coat hanger to break the sludge loose from inside the fitting, reconnected everything and tried my nifty new nozzle tool again. Worked like a champ. Flushed the vent line and pumped out and flushed the holding tank. System works great. Terry