I see you've just joined our merry band of boat
You might start by replacing that "paper thin" bulkhead with something thick enough (1/2" would be thick enough) to let you cut a proper access hatch
in it--maybe even a couple of hatches if necessary to provide access to both ends of the tank or maybe a macerator pump
or vent line--which you can trim with something like picture frame moulding if cosmetics matter, along with thin sliding panels
to cover the openings. You decide where and what what size they need to be.
Just a "top of the head
idea...others may have better ones.
As for why Beneteau
designed it that way...boat builders rarely if ever DESIGN sanitation systems. To them sanitation systems just add cost that contribute nothing to the value of the boat
. So they put them together as cheaply as possible without regard for the fact that, unlike water
, sewage must be managed and count on the problems that arise from their failure to acknowledge this waiting until after the warranty has expired to make themselves known.