I've been waiting for someone who has been hands-on with this to help you out.. in the meantime, do you have the installation manual
? (That might not be the exact model however).
I believe the wax ring under the black tank is to keep shower water from going out the big hole where the black tank drain is, and maybe to keep bugs from coming in. I dont think it has anything to do with sealing the black tank. That is done with the drain pipe connection.
Are you sure the water is coming from the black water tank and not the shower pan? If so then I expect it will require removal of the toilet, then removal of the black water tank, and reinstalling all according to the manual (using a wax ring in place of caulk in the floor area as described in the manual.) Take notes as you disassemble and it should all make sense.
In fact even if it is only shower pan water, I guess you still have to remove the toilet and tank to replace the wax ring... or just put a bucket under it when showering. But then eventually the floor will rot also.. Glad I don't use my shower!