When we bought our 27 (1988) the holding tank hose flanges were pretty well gone and the tank was too small. Here is a picture of the replacement tank. I built it from 12 gauge 304 stainless. The outboard side of the tank follows the contour of the hull. It has a macerator pump, diversion valve, and tank flusher. The system is capable of Direct thru hull, holding tank, deck pump out, and thru hull macerator pump out. It works out great for cruising where pump out is not available or needed. While we have never used the direct thru hull, it is there if we ever go offshore. It took a while to figure this out, so maybe someone can use the design. If anyone is interested, I can take more pics and describe the internal baffles and tank flusher.
(http://i1054.photobucket.com/albums/s498/bobndenice/IMG_3855_zpsf7e4d180.jpg) (http://s1054.photobucket.com/user/bobndenice/media/IMG_3855_zpsf7e4d180.jpg.html)
(http://i1054.photobucket.com/albums/s498/bobndenice/IMG_3854_zps28dd9fa0.jpg) (http://s1054.photobucket.com/user/bobndenice/media/IMG_3854_zps28dd9fa0.jpg.html)
Great idea and needed where we live. The inability to empty tank when offshore is in addition to the problem of having and entirely too small tank, especially if your wife is on the boat for a week. Seems like we pumped out every other day. We only use fresh water in the head.