I though most RVs now had flush systems? We fill the tanks with flush system on and let it flush for five minutes after the black empties and refill it with a little water maybe five gallons. Then we dump the gray tank and flush further five minutes and put a couple gallons back in.
We add water softener to the black tank along with the chemicals.
Putting some water in the empty tank and chemicals will assure that everything gets broken up, if it isn't, going down the road. The soap from sink and shower is the last thing through the hose then clear rinse. The hose doesn't smell when stored nor do the tanks. Those without a flush system might stick a hose for black tank only past the valve, in the toilet as the black drains and for a bit after. They sell a wand that you stick in below the toilet, on a hose, tp help clear things up. Then there is always the five lbs or so of ice and five gallons of water trick and drive down the road to break up things.
If it is venting then check to see if you have broken stack or a nest or something in the top. If you are flushing the toilet with vent fan on there could be your problem. The units are rather air tight and you suck all the odors from the tank in.
