Hello Scubado. You appear to have got some confusing answers. On the lake, with no overboard discharge, you do not need an anti-syphon vent. The toilet outlet can be below the tank inlet, mine is 18 inches below, and has been for the past 28 years with no trouble at all. The toilet hand pump can push the effluent upwards of three feet above the toilet outlet, provided the piston seal does not leak.