Hi griffly16,
Firstly, identify the exact model of your toilet - on labels on both the fixed and the cassette parts.
Head to
Thetford Support and choose, in turn, the Spare Parts breakdown for your specific model, and the repair instructions, again for your specific model, but choose the tasks which you are likely to need. (Tip, download the lot into a local folder, then you have them as and when you need them).
There is lots of information there, most of it with helpful pictures.
From your posts I would guess that there is a valve near to the pump which does not close properly. Cleaning is usually a good fix.
Don't use a spray oil such as WD40, you need a silicone based lubricant for toilet parts so that you do not harm the seals.
Best opf luck, may you soon have a toilet where the wet is only where intended.