This article is how I removed and replaced my leaking aluminum holding tank and replaced with a poly tank.
First, order your new tank, SMALLER than the specification that Hunter provides. you will never get the new tank into the hole that is removed. I had to have my new tank cut down 2 in to get it into the hull. Make your cuts at the shower as indicated in the picture and as provided by Hunter with a vibrating saw from Harbor Freight. This entails removing two Teak slats on the top of the shower seat. Cut the plumbing lines attached to the tank, under the bathroom Sink. You will then a saw to cut out the old aluminum tank. I used a reciprocating saw as well as a roto zip circular saw. You will have to pull and tug to get it out which is the toughest part of the job. Once this all out, insert the new tank thru the front of shower seat opening. Spray expanding insulation from Home Depot under and behind the new tank to get it to stay in place and be insulated from the hull of the boat. I added a cut off ball valve at the base of the tank under the sink, as well as piece of pvc drain so when the tank has some contents, when heeling over, the sewage will sit in the PVC section rather than actually on the new drainage tubing. This will help keep saturating the plumbing lines and keep the lines from smelling after time. Run the plumbing lines as before, or as necessary for a manual pump out or macerator pump( I did nether just because of the limited space) and the line from the toilet. Once your ready to reseal the shower front piece you removed, I placed aluminum strips from Home depot on the edges of the fiberglass and secured them all down the self tapping small screws to hold that piece in place. I then used Marine Tex to fill over the approximated edges, feathering it out like float/tape, to a smooth finish. Once this was all dry, I then painted the front with the exact same color (sand) I got from West Marine. I now have a tank, all new plumbing, and repair that is hardly noticeable at all.