Ive only ever used no-name Woolies Laundry Soaker (NappySan knock off). Identical ingredient to Vanish NappySan but less than 1/3 the price. I never touch the over priced and over rated caravan shop liquids. The active ingredient in all nappy soakers is Sodium Percarbonate and the no-name variety always has a higher percentage, than the branded offerings. One heaped cap full in to a litre or so of water in the bottom tank, but like the expensive chemicals only put it in when your just about to use the can. The active ingredients only work for a couple of days, so if you put it in a couple of days too early the toilet the chemicals will have expired by the time you need them. Make sure you empty the toilet on 3 day at most, and every second day preferably.
Every time I empty the can, I rinse it out and wash it out by half filling it, and using some sink detergent shaking it about gently. Then rinse a couple of times, Then leave a litre or so of clean water (without chemicals) ready for the next battle.
We don't generally never have a stinky dunny. The key is don't let it ferment for more than a coupe of days