I would agree that the most trouble-free method for a happy Thetford is to use plenty of water to wash out the cassette. And I know this next bit is slightly off-topic, but PLEASE folks, don't put any wet wipes or sanitary products of any sort in the cassette or down your loo at home! The manufacturers of these products always claim they are flushable, and technically they are, but they do not get broken down in the sewage treatment works, and end up blocking filters, or even worse, causing pollution in the sea. Just ask the organisation "Surfers Against Sewage"! (Or any of the environmental groups.)
Ok, I'll get off my soap box now... happy holidays everyone!