Hi David
I also want to use a dry/compost toilet on my boat and RV. I? had a look at your explanations and picture and it looks like quite a simple DIY project. The disposal of urine is the main issue with those system. You said:?"The fact that the urine drains directly overboard helped" seems to solve that problem, at least for a boat. Could you please explain how you did this system. From you picture the head looks pretty low. Do you use a bilge pump of some kind to bring the liquid at a level that is safe above the waterline?
Alain Dupuis