A friend had the same problem. discovered that the vent pipe was installed too low in the black tank and the when the level in the tank got above the pipe, there was no place for the air (and odors) to vent to. Another symptom he noticed was that when flushing the toilet, it would "burb" air when flushing.
Jerry and Irene
2015 Chevy High Country 3500 CC/LB D/A -- 2009 Cedar Creek 34 TSA
2012 - nights camped - 153; miles on CC - 14779
2013 - nights camped - 88; miles on CC - 9066
2014 - nights camped - 57; miles on CC - 5263
2015 - nights camped - 95; miles on CC - 11179