I would highly recommend ditching that for a solid holding tank. First, the tank's contents are going to stink, since there is no vent. Second, if it springs a leak, you'll really be wishing you had switched it out for a solid holding tank. Third, I can't see how the fuel bladder isn't going to eventually permeate and stink like all hades. Fourth, you really want a deck-mounted pumpout fitting.
All good advice. However, I just don't see a place for a solid holding tank. The freshwater tank takes up most of the room, and the bladder is just fitted in the leftover space.
I will see about moving that fitting, though. There's got to be a way.