On my cassette I can remove the entire internal workings attached to a cover plate which can be rotated and lifted out. It is then easy to clean the internal workings and the cassette with a mix of soapy water and bleach then leave open until dryed out.
As Don has said, it should be possible to gain access to the cassette’s interior fairly easily, but that should not be necessary for simple internal cleaning. (I would suggest that - unless there’s something wrong with the cassette’s operating mechanism or seals - dismantling should be avoided.)
Thetford’s advice nowadays would (unsurprisingly) be to use their own cleaning product. Historically Thetford recommended a citric acid solution (which is what I use) but alternatve methods have been found to be effective. For example
(It goes without saying that the more frequently the toilet is used and the less regularly cassette-cleaning is carried out, the greater the build up of ‘scale’ will be within the cassette.)
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