To the OP, sorry to here your woes,All I can say is Air head - really. Skip the tank and start composting - no float valve/switch/pump out required.
We just installed a natures head, never ever want to go through the inevitable horror of playing the failed marine head game, I have read far to many threads/post about the failures, the aroma, etc, I just don't see the appeal.
I do realize that many folk have success with their marine heads, but I suspect that composting heads will become more and more common place on boats, they make to much sense. Just one mans opinion.