The black tank and toilet smelled terrible when I bought my used 5th wheel. I discovered that I had the dreaded poop pyramid, the previous owner obviously had little experience with an RV tank. So the first thing I did was empty the tank as best I could, I was in an RV Park with hookups, so I was able to dump....then I filled the tank with a full hot water tank of hot water....and added some dawn dishwashing liquid, not too much just a good long squirt. Went to bed.
The next morning I was traveling so I emptied the hot water heater again in the toilet. Now the blackwater tank was almost 3/4 full. I pulled out and stopped at the park office and bought two large bags of ice. I dumped both bags of ice in the toilet and then drove for about 100 miles....when I arrived at the park I hooked up and dumped the blackwater tank, then I filled the tank with another hot water heater full of hot water, another squirt of Dawn dishwashing liquid and then filled the tank back up to 3/4 and dumped it again.
It has not smelled since in two years and I use plenty of water when flushing and only empty when at least 3/4 full. I never leave the back valve open when on sewer hook-ups, I actually keep both my grey and blackwater closed and dump when nearly full. I leave the hose hooked up but the gate valves closed. I dump the black first and when that is finished I dump the grey to flush the hose out.
I have never had the need to use any chemicals and doubt I ever will. I can even leave the fan running when I flush and don't get an odor, at least nothing worse than what I just put in!!!
I have a four flush rule in my RV. Fill the bowl, do your business, flush and fill the bowl three more times and flush.