Never seen an adapter from a cassette direct to sewer. I will say, that when we had a cassette in our old PUP, it was easy to clean/flush. Not messy at all. Good seals, good mechanisms. Kept it when we traded the PUP (whyI don't know!). It now sits on a shelf with other no-longer-used camping gear....
2014 RAM 2500 Laramie 4x4 Quad Cab w/Cummins Turbo Diesel, 2009 Rockwood 2607, and 3 'yaks!
300W of Suntech Solar, a Rogue MPT-3024 Controller, and a Xantrex PW2000 Inverter