Does this have a rubber seal?
The Thetford Porta Potti dump cap does come with a seal fitted already.
To prolong the life of your seal you can smear it with olive oil every now and then to keep the rubber supple.
Although these seals do not last forever, or you loose your seal you can buy the replacement seal. The Thetford seal pack comes as a Thetford and Cap & Spout Seal, the part number for this is 16195.