I would suggest taking out the toilet and installing a nice flushing
portable. That's what I did
It has a 5 gallon holding tank
and a built in water
reservoir for flushing
. I use a bio treatment and have not had any smell or issues after 2 months of use (it's not even half full).
Or you could try the composting toilet route
. Its more expensive than a portable, but probably the same ultimate cost as a holding tank
retrofit, and the end by-product in an environmentally friend/favorable compost.
I would shy away from a holding tank, unless it already came with the boat. I have seen some unpleasant (to put it mildly) re-fits in the past. I say go cheap
, go easy, so you can sail now
Cheers and Happy Sailing!