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My first Morello had a Thetford toilet and we had no problems in 13 years, it never leaked and I did not do any maintenance. The new Morello has a dometic cassette toilet which has given recurrent problems with leaking. It can be dismantled and silicon grease applied to the seals which improves matters for a time.
I attach the dometic manual, which recommends very frequent re-greasing:
dometic-toilet-operating_manual_55425