Black bits in the loo flush

If this has been referred to before the apologies but couldnt find anything.

Am sure Im not alone here, but how do I stop getting black bits of (mould ?) coming through the loo flush? We dont take the van out every month, but every time we do, it seems to take ages to get rid. Someone suggested I always keep the flush topped up even when not using the van as this will stop it - did that and it doesnt. Tried various types of 'toilet flush liquids' but still seem to get the same problem.

Am I doing something wrong or does anyone have any suggestions as to how to eliminate ?

Many thanks