I suspect the 'full' light is activated by a float arm inside the holding tank, as is the red/green indicator on th emanual version. You should be able to test this by removing the holding tank, looking inside the housing and finding a small arm holding a small cylinderical magnet. Move this back and forwards while having some one watch the indicator lamp. If this works OK thenthe fault will be with the 'sender' in the holding tank itself. If not, then check out fuse and light itself.
The sender in the tank is a float arm also holding a magnet. If you have cleaned out the tank very vigoursly then it is possile that the 'float' part has become detached and possibly flushed away. Look at the thetford website - from memory
www.thetford.com and you will find service instructions, although re-fitting a float arm inside the tank is none too easy.