My in-laws have a cottage in Traverse City, MI. It’s fully piped. Every year my hubby and his brother blow out the pipes and put RV antifreeze in the toilets. No problem with the plumbing at all.
We bought two rainbarrels a couple years ago. One is in use right now. We use it for outdoor watering all summer. Your gravity fed system is a great idea.
I saw a “Dirty Jobs” where Mike Rowe was making huge water containers for buildings in NYC. They have gravity systems. I wonder how a rainbarrel would fit into that equation for a smaller building.
Gravity-fed systems work fine - one of the ealiest examples of plumbing-on-demand.
Your toiletette works that way, now. Just need a water supplu above the john.