I was just browsing and cam e across this composting head its made for 12 volt for a weekender
But to me it could be fitted to a boat
What I like about it is urine could be either directed over the side or collected
http://www.abetterwaytogo.com.au/products/separett-weekender.aspx?lv.crumb=17384 
I dont think its overly large like some designs
