I think you might find this water is actually rainwater that has got in via the hatch door seal. I have found these doors do not always seal very well, and due to the internal profile, if rain gets round the seal, it easily gets across the threshold of the van wall. I had wet carpet around the rear of a C200 toilet - immediately ready to blame the cassette unit, but found that this was coming in from outside! If you look closely at the way the door closes, and relies very much on compressing a flimsy rubber gasket, you may spot something. Also if you examine the fittings within the rear of the toilet, I think you will find that it is pretty unlikely for the flush tank to leak anywhere. If it is the hatch, try some self adhesive (waterproof) draught excluder around the inner frame, this might provide sufficient sealing for you. Good luck! Richard.