recent change out
I recently bought a 1977 Ericson, and in the purchase, got much more than the boat, the previous owner apparently didn't know the holding tank could actually be pumped out.
Anyway, I went back and forth with upgrades, and in a perfect world, probably would've bought the composting toilet by AirHead. I spoke with the owner of that company, and after many measurements, photos and head scratching, turned out that their system wouldn't fit my 32-2 existing set up in the head.
I opted for the cheaper solution, which has 100% removed the smell from 40 years of neglect:
1)remove the old system as suggested, with a pair of gloves, holding your breath and cutting out the old lines.
2) if you can salvage your existing tank and get it cleaned to your liking, save this
3) replace existing tank if necessary
4) repaint the interior hatch where that tank has been stored, it's no doubt had a leak and over time, the smell has escaped the hold
5) replace with new lines
6) I've initiated a new rule that #2 happens only in an emergency (and gets pumped immediately at the next opportunity)
7) My plan is that if I keep up with it, and flush it regularly, the smell won't be a problem (a friend also suggested flushing it with vinegar after a flush)
8) I now pump out, then run a hose into the pump out hole, twice filling the tank and pumping out again. This assures me that ONLY fresh water is left on the bottom of the tank
Good luck with the upgrades!