Positive it has one. Clearly marked "Black Tank Flush". I can follow the line to under the bathroom sink, I have tested the backflow preventer and followed the line under the shower and back behind the wet bay. The line goes forward towards the black tank which sits between the rear wheels / above the fuel tank. The inside of the tank is "cruddy" with years of neglect.
Since I can't see the flush nozzle with a camera inside the tank I get the unenviable task of pressure washing inside the tank and hope I hit the flush nozzle enough to clean it - wherever it is.
I may opt to cap it off and add a new nozzle. Even though it will be on the wrong end of the tank to push anything out, it will help clean the surfaces.