I quit using chemicals in my holding tank about 4 years ago. I get no odors inside the camper. When flushing, the flapper is only open for a few seconds, and a deluge of water is going down the hole, so very little gas can escape from the holding tank. Most of it probably goes up the vent stack, as it should.
I also never clean my holding tank in any way. I simply fill the holding tank to maximum with water at the end of my trip, so that when I open the waste gate I get a "mega flush" pushing all the contents down the poop pipe.
As far as the smell when dumping, unless you are spilling sewage when dumping there should be no smells. The dump station at my storage lot has 2 dump entries - one a threaded PVC cleanout which I use with my threaded adapter, but there is also a traditional "flapper" hole in a cement bowl for people who only have a "elbow in the hole" type of dump.
If you are using the old "elbow in the hole" type of dump then you might have some smells escape. But I always use my threaded adapter so there is no leakage when I dump. Sometimes you get a whiff of gas coming out the "flapper" hole while I'm dumping down the clean-out hole, but oh well. You'll never smell it more than 5 feet away and in 10 years of RVing I've never once had anyone waiting behind me while I dump anyway.
It's cheaper not to use any chemicals, and it's better for the environment.