First thing is to figure out which tank the sewer smell is coming from as gray can smell as bad or worse than black. I would dump the black tank and then go inside the trailer and see if the smell is present. If no smell then dump the gray tank and go check for smell.
If it's the black tank that smells, make sure there is water in the bowl and the roof vent is closed while dumping.
If it is the gray tank that smells, you will probably want to check or replace all the air admittance valves under the cabinets.
LinkAre you running a fan of some sort while dumping that could creating a vacuum inside the trailer? That could cause odors to be sucked in while dumping.