Packets or Liquid for Black water tank

I use nothing, and haven't had serious issues with odors. IMO many times odors are caused by a poor seal at the base of the toilet, or failure of the seal inside the toilet. Occasionally also odd wind patterns that push air down the vent pipe, but that's pretty rare in my experience.

Also make sure there's some water in the trap for your tub and sinks. If it gets into the trailer, the gray tank can stink too.