I never had an issue with my previous trailer. I've been having black tank smell with my current trailer, and just (sorta) solved it yesterday. For some reason, the smells were coming through the hole that the toilet fresh water is coming through. I plugged that hole and the smell is gone. It doesn't explain why it's coming through the hole. I suspect that the black tank vent isn't sealed to the tank.
Things to check:
* Make sure that the flush valve holds water
* Make sure that your vent isn't blocked
* Make sure the vent is sealed to the tank
* Make sure that the seal between the toilet and the floor is good.
* Make sure that the seal where foot pedal or the hand flush is sealed where it enters the tank.
* Make sure your dump valve is always closed, with a couple of gallons of water put back in before use.
When it is all sealed properly, I found that I don't need any additives at all.
I have no experience with this, but I hear that the grey water tank can often be the source of smells in the bathroom. Make sure that your P-Traps have water. If the smell is coming from under the sink, your air admittance valves might need to be replaced.