I am very excited to see that you will be using a composting toilet in a wet bath. I'm remodeling a '74 Tradewind with a complete new floorplan and plan on doing exactly the same! LOVE the concept of composting toilets. Which model will you be using? I emailed Nature's Head a while back and they confirmed that you can use theirs in a wet bath, just drill some drain holes in the liquid container HOLDER (not the bottle -
Also, I have an inexpensive Ikea foam mattress that I cut to size and topped with a 3" memory foam topper - very comfy!