Could be cracked tank or if you have a fold over basin that is situated over the toilet it could be the empty trough that as you tip the basin up to it's store position the water leaks out through mastic seals.
If you basin IS the fold up type over the toilet then fill it with water and tip it and see if that is causing the leak on to the carpet if it is remove the sink usually by lifting and pulling towards you in it's open position. Clean and reseal the mastic joints.
If the modern type of toilet it could be something else so check with thetford on www.thetford.com 'contact us' top line to right click on it and fill in the email form.
Rob
July 2008