Go to www.thetford-europe.com look for service top line click repair instruction on next page to appear. then sanitation then down load the pdf file it covers all range of toilets. However the fault will not be on there so go back and look for 'ask thetford' using their online form ask them what could be the cause.
If intermittent then it could be the micro switches. if it was the main 12volt switch faulty you would get same on all 12volt systems so it has to be a short between the toilet switch and the point the wire picks up current. The other theory a dodgy fuse which is in the cassette housing see the repair instructions for its location. 3amp ...no higher.
You could ask thetford by looking for
'Ask thetford in their listings. fill in their online form.