We use the natural bacteria additive. The brand I like is called Odorless. It's basically the same bacteria that the municipal sewer plants use. It feeds on the gunk you put down the toilet. I dump my tanks about every five days. We use lots of water when flushing. I always flush the tank when I dump (I have a tank rinser on my rig). We add the bacteria additive after flushing and dumping.
It works better than the chemicals, it's cheap, and it's safer. The odors in the summer will eventually come, that's why we dump every five days. We have a family of five, so lots gets dropped in that tank.