Sandra, regardless of which CDory you choose, consider installing a composting toilet instead of a Marine head with holding tank. Our new dirt boat (Oliver travel trailer) has a Nature’s Head, and it is so much cleaner and simpler than having to dump or pump out a holding tank. Liquids are diverted to a small tank, and solids to the composting hopper. There is no foul odor, and what odor is there (a slight coconut smell from the coconut coir in the solids hopper) is vented to the outside via a small fan and vent tube.
You might talk to Pat and Patty Anderson about their experiences with a composting toilet on their CD 25.