Re: How big a holding tank vs. camping
If your head has a sink then it is trivial to do a fresh water flush. Just leave the toilet always turned to "dry" and fill a container from the sink then pour that into the head. We keep an old pot behind the toilet for this purpose. I always do my last flush of the day with fresh water, and when sailing near a constant water supply we also use fresh water.
My old Catalina 25 had an even nicer option. The drain from the head sink went to the same through hull as the water inlet of the toilet. We would close the through hull and then you could fill the sink with fresh water and when you pumped the toilet it would draw that "grey water" into the toilet for flushing.
boats that I sail on regularly: Pearson 28-2, Rondar 505, Yankee 30, Blanchard Junior