My wife feels the need to use chemicals, so we buy something cheap at Walmart. Half the time we don't use anything, and if she asks me I tell her I used something. Neither one of us can tell a difference anyways.
I've found that turning the fan OFF while flushing is the single biggest thing you can do to keep odors down. We have a fantastic fan upgrade on our trailer and that sucker moves air!!! Fan off, flush properly and no odors.
To piggyback on what others said... your holding tanks are NOT septic systems. They were never designed to be part of the breakdown process. They are simply a sealed holding system for waste until you can dump it.