For a Cal 9.2, I had a bladder tank under the V-berth. Mid summer was not very pleasant, we have limited pumpout facility access. I fabricated a 3/8 to 1/2" thick tank out of
epoxy resin and fiberglass, sanding the inside smooth with 40 grit and poured a slick coat on all surfaces, also poured a little in every corner. Its been a couple yrs now and the stench is gone. I used the eternal level sensor that you lay down the 2 foil traces trimming the bottom for "0". It works great and no extra holes in the top.
It fits right behind the head so the hoses are as short as possible.
http://www.angelfire.com/tn/santana5...ding_tank2.JPG