There is no wax ring, but there is a soft seal that I HIGHLY recommend you use. My TT didn' t have one, and every time I flushed the toitey I'd get some leakage at the base of the unit. When I asked at the RV place about that, they told me about the soft seal and indicated that it should have been installed when the built the trailer.
If you go to the Thetford website and look at the drawings, you can see the soft seal included with the flange bolt kit, it is ITEM #5 ON THIS PAGE
The piece looks like THIS THING