I had the same problem. I have a rear holding tank as well as a front holding tank. The cutoff valve for the rear tank is close to where the tank is located with the PVC running half the length of the trailer to the point where the sewer hose connects in front of the axles . When I empty the tanks, water accumulates in the PVC between the rear cutoff valve and where the hose connects at the front. The next time I remove the cap to connect the hose the water that was left in the PVC runs out. To drain all the water including the residual in the PVC from the rear tank I lower the landing jack legs so the water will run out of the PVC. Poor design but not the dealer or manufacturers problem once you own the trailer.
