I don't know exactly which model the TM's now use, but ours is very much like this one:
http://www.thetford.com/Home/Product...6/Default.aspx
If you read through the owners manual you'll get an idea of what it is all about.
Basically, there is no black water tank on TM's, so the tank is within the toilet itself. It flushes by "recirculating" the filtered liquid in the tank as the flush water. You're supposed to "charge" the toilet first with fresh water and add approved chemicals to make the toilet stay fresh-smelling. You only see liquid in the bowl when it's being flushed (or when it gets too full!).
Hope this helps.
D