I had the same leak on my Thetford Aqua Magic toilet. There is a valve that controls the water flow and mine began to leak. To service it you must remove the toilet, very easy. Remove two nuts holding the toilet down and disconnect the water line, also easy. You will then find the valve. You can replace it for about $30. In my case I found the valve stuck to the plastic tubing so I looked closer. There is an O ring that seals the shaft of the valve and I just got one out of my assortment of O rings and replaced it. The leak has not reoccurred for several years. The valve is about the only thing that can leak. Two things to check first are the the water connection is tightened and the toilet is tightly secured to the floor.