I don't know what some people are putting into their black tank but what I put in there does not smell like flowers going in. I mean let's be real. The black tank will not magically make something that stinks, sweet. AAMOF the smell, if left untreated, goes downhill after going into the holding tank. This means that we can only minimize the impact.
To do this we have to use some sort of cleaning regimen PLUS a deodorizer. Proper flushing with prompt treatment with the product of your choice will work to the degree that you are going to accept. It's that simple. Some like the Geo method, some prefer chemicals. There is no one answer. If there were a universal solution there would be no discussion. ;)
OP - I suggest you ensure your tank vents are not blocked, then use a good digester with deodorizer. Also, when flushing ensure the vent fan is off while the flapper valve is open. When dumping ensure there is a good seal between the end of the stinky slinky and the hole in the ground. Also be fastidious with rinsing out the tank after dumping, then prime the tanks with the "stuff of choice" immediately, whether or not the trailer is still being used afterwards. I do all of that and have no smells.