I have been following this thread on Caravan Talk:-
From about the 4th post down there is mention of an adaptation available from Thetford to alleviate the problem of water freezing in the flush tank U-bend,basically fitting a drain tube.
It is said that this only affects toilets manufactured after April 2010 but looking at the photo's the "problem" pipework looks very similar to mine,a free pipe for home fitting is available from Thetford if it is.
If you e-mail Thetford with the asked for details in the link above they will tell you if your toilet is affected and send you a spare part if it is.
Next time I go to storage I will check ours.