You could put a couple of drops of food coloring in half a bowl full of water, and figure it out in a minute.
So, it depends. If it's "used" water (to put it gently), it's probably the gasket between the toilet base and the tank. Actually, there are a couple of gaskets that could leak and mess you up. Replacement gaskets are available at your friendly RV dealer, but there are two different kinds of Thetfords, before 1985 and after 1985, so you'll basically need to take the toilet off and apart before you go get the new gaskets.
If it's "first time through" water, there are different options. Either the water inlet valve is leaking, or the internal vinyl hoses have basically died on you.
Mine was leaking clean water, and it turned out to be the hoses. You'll need about 6 feet of vinyl tubing and a pair of hose clamp pliers like these.
Don't even think about doing it without the pliers - trust me on this!