The boat only has a Porta-Potty, and the remnants of and old macerator system. I want to put in a real head with Y-valve and holding tank, but can't figure out where to put everything. One of the other R-33s I looked at had a flexible holding tank in the forward salon lower compartment port side, but the thing sat over the speed and depth transducers, making them inaccessible. That to me is unsat.
First, consider a composting head like the Air Head or Nature's Head to avoid this problem altogether. I'm taking out my tank and doing this.
My R33 had a ~20 gallon holding tank in the port cockpit locker, greatly reducing the storage there, but also making the "floor" higher so things didn't disappear toward the shaft. My transducers are under the v-berth, so a tank in the salon wouldn't be a problem in that case. And if your transducers are in the salon, maybe put a flexible tank in the v-berth. It's kind of a vertical space, but a decent size. Directly under the cockpit sole seems like a good spot for a rectangular tank.