Theres a dead easy way of dealing with it....rip the bloody thing out and fit a proper RV toilet with holding tank and gate valve waste point under the motorhome.
Or you could try a cork
Your in luck Digger,
We carry a lot of Tesco, Lidl shopping plactic bags. In 40 years of Wild Camping, we have never ever used our toilets for"Long". Only for "Short".
Toilets have a 2 lids. One to cover and the other to sit on. Lift up both, place a shopping bag in the recess making sure the edges of the bag are around the rim. Then drop the seating lid. When finished, tie a knot at top of bag and place it legally where ever you are. This way there are no unwanted smells for you or your partner.