The one time we had this problem it was due to the black tank vent being partially blocked. To check that the vent is open you have to get on the roof of the trailer, remove the vent cover and look down the vent tube with a flashing. You should be able to see into the black tank. Another issue could be the vent cover has shifted and is blocking the vent. I cut about 1/4 - 1/2 inch from the top of the vent to provide more space for fumes from the tank to escape and that solved the problem. If that vent is blocked gas pressure builds up in the black tank and when you flush the toilet the pressure and fumes get released into the trailer.
