Re: How big a holding tank vs. camping
Here's what my bareboat sailing instructor taught me more than 10 years ago. It's the method I share to this very day whenever on a boat. It's received well because it works well.
Pee: Dry bowl (NO lake water intake). After going, fill a cup with fresh water from the sink. Pour water, using quick swirl motion, into bowl. Pump bowl dry. That's it. This eliminates much of the volume going into the tank--pumping lake water into the bowl. Folks don't realize how much water this pushes into the tank.
Poop: Follow same method as above. HOWEVER, add one extra step; before going, be sure to pour a cup of water into the bowl. If necessary, swirl a second cup through the bowl after going.
Using this manner, 3-4 adult males can make a ~25 gallon tank last the better part of a week. When tied up in the evenings, use shore facilities. While at sea, guys can pee off the back of the boat.
We once had a girl at the helm. A guy peed of the back and she had no clue.
My personal favorite is to sit at lifelines--feet dangling over the side "rail meat" style. If not heeling too much, then do this on the low side (unorthodox but works better than high side). Pee out the bottom of your shorts. NOBODY, and I mean nobody will know, except for the "tinkling" sound in calmer conditions. You can be staring straight into the eyes of someone in the cockpit. They cannot see your lower half because you're forward enough that the cabin top blocks their view!
Don't ask me how I know; "I heard this from a friend".