sounds like microswitch is stuck.
www.thetford.com Uk
www.thetford-europe.com UK/Europe
www.thetford.com.au ( for those in Australia)
find your model under permanent toilets and you should see the free repair pdf link below the picture. If no luck search on site for 'Ask Thetford' send them message on their online form.
bob lunar solar owner, April 2012