As above. The pink stuff is the source of your problem, lots of discussion about this on the C&MC site. Rinse out your flush tank until the black stuff has gone and do not use the pink stuff again.
Some say you do not need a flush additive, saying you do not bother with your toilet at home so why do so in the caravan. I don't think this argument stands up as a home wc uses several litre of water to rinse the bowl as opposed to a couple of squirts in the caravan.
We turned to Elsan Green many years ago. 75ml and 2 L of water in the toilet waste tank and a squirt in the flush tank, just enough to colour the water slightly, works a treat. Loo is clean and free of smells even in high 30's after 4 days when we usually empty it.
Colin