Go to www.thetford.europe.com.
Model, if known, would have helped.
Either Select your model by viewing pictures of type it is
Contact Thetford using online form ask what could be cause.
Meantime try this on the cassette is a rubber seal around the blade opening.
This may be letting flush water to seap past when blade is opened and bowl contebt released and flushed.
To test: talc or chalk powder. Spread around the edge of the rubber seal about 1/4 from it. Replace the cassette 1/4 fill the bowl with water, leave four 15mins open the blade if any water remains, CLOSE the blade then remove cassette wet powder means its likely either wrong seal(they are year dated according to caravan age and toilet model)or or the sea is old and not doing its job. Always, always check cassette blade is CLOSED before any attempt to remove cassette from housing or you'll snap the blade operating arm.
Bob Uk
December 2016