Chemicals that mask odor, ultimately cause odor. Not to mention, they have to be bad for the environment, wherever you ultimately dispose of your holding tank. Even a pump out ends up in the ground somewhere.
The concept is to maintain a healthy tank, with plenty of oxygen and nothing that will kill the good aerobic bacteria that break down waste and do not smell. Oil, chemicals and lack of oxygen are what kill these good bacteria and allow the anaerobic bacteria, which we all relate to smelly sewage, to thrive.
I've never had trouble flushing TP, but you have to use it sparingly. Most critically, you have to flush a full bowl of clear water behind your waste to push it all through the piping and into your tank. Too many just think its gone, when out of the bowl, but the paper is really sitting in the line behind your head and will settle back down to the bottom, if you didn't push it through. This is the cause of most cloggings, not using too much in the first place.
I supplemented by tank with KO for years, but switched to Happy Camper last season. It's substantially less expensive, however, I find it better to mix with water first. KO comes as a liquid. I also recall KO had an expiration date, but I haven't seen one on HC.
I also clean the bowls with CP cleaner, which won't kill tank bacteria. I bet many clean with a bleach based cleaner, which would destroy bacteria on the next flush. We use bleach on the walls, counter tops, etc, but never in the bowl.