Toilet bowl fills with flush fluid

[tankdfm]

Every so often (2 or 3 months) I'll go to use my Thetford C250 and the bowl will be full of flush fluid. The first time I assumed it was a faulty flush button so I changed that it happened again. Next I removed the fuse for the full light still did not solve.
Then I thought it could be siphoning so I put less in the flush holding tank and I thought this had solved the issue but this morning I awoke to a full bowl again.
Any Ideas please.
Thanks

 

[HenryB1]

I had this problem from new with my last 2010 Bailey Pegasus. In the end I realised that it was happening after either heavy or prolonged rainfall. My dealer checked the external filler cap for the flush tank and it was not sealing properly and was allowing rain into the flush tank that was overflowing into the toilet bowl. The external filler cap was replaced under warranty and I never had the problem again.

 

[tankdfm]

Thanks for that HenryB1 but was not my issue unfortunately as it has only happens when I'm using the caravan and I always have the awning up which keeps any rain off the filler. I'm now keeping a diary to see if there is any pattern to it.