I have had occasional problems with the "joker" valve in a Jabsco head allowing waste from the waste exit line flow back into the head. (not a lot of volume, but it creates a stink) The waste is coming from the vent loop which, on my boat, goes up about 2.5' above the level of the joker valve. I believe that an entire tank of waste above the head creates the potential for a disaster, as a failure of the joker valve would allow the entire contents of the tank to empty back into the head... ugh!!
Hope this helps,