We had the same problem - trouble shot it by trying to completely fill the grey water tank from the kitchen sink. Found a crack in the inlet abs pipe that allowed water to leak out under the 5th wheel just forward of the front wheel on the door side (only when it was "overfilled"). Since that grey water can actually smell worse than the black water holding tank, those fumes were drawn out when we used the vent fans. We replaced the broken pipe, and no more odors!!! Nothing a few $$$$$ couldn't fix!! Good luck with the problem.