I have a porta pottie aboard my O''Day 23 and it has worked very well for me. My wife and I spend weekends on the local bays and use the porta pottie as a normal toilet and we normally only use half of the 3 gal. holding tank. If you are going to use a porta pottie, I have a couple of suggestions. First, use silicon grease around the lid gaskets and the sliding flush mechanism. I have found that this has greatly reduced accidental spillage taking it apart and dumping it. Get the type of grease used for SCUBA gear and DO NOT use a petroleum gel as it may break-down the seals. Second, I treat the sewage in the holding tank with a chemical made for porta potties.
Not much mess and it smells a whole lot better than my grandson''s dirty diaper!