I still have the porta-pottie but was looking at..
...the SeaLand Marine Traveler. It's a combined ceramic head and holding tank (9 gal) so plumbing is minimal just pump-out and vent. Since it does not require pump-through of waste through plumbing, most of the tank would be waste. Single action dump and flush with hand or foot. It would just fit where the porta-pottie is. If anyone has used this let me know. alan