A V shaped tank under the V-Berth would be the most logical solution for most people but for live-aboards' like myself it wont work. My tools take up all that space.
While thinking about this I just had another idea.
OK, You can buy a bladder tank and fabricate a plywood
box around it with a lid. That would keep any odors at bay.
The next problem would be to transfer the waste from the Traveler tanks
to the bladder tank. This could be moved with the macerator pump but you would have to monitor
the bladder tank before the transfer so as not to overfill the bladder. If you have 2 9 gal Traveler tanks and a 20 gal bladder tank, that would give you 38 gal. capacity total. that should easily handle two people for a full week.
The only reason I wont fabricate fiberglass
down there is because there would be no safe way to work with the fumes unless you had one of those breathing outfits with supplied air because you cant filter the resin fumes.
Another thought was to remove the travelers and make a new tank under the head
but about twice as high. That would give you approx. two 15 gal tanks - one for each head for a total of 30 gals. that's getting more to a respectable amount of sewage. That would raise the potty seat substantially so a new foot rest would have to be made.