your right. the porta potties with pump out feature pump right out of the potties tank, no seperate tank. Vacu flush has a holding tank. Vacu-flush is expensive. I installed an electric head on mine that chews eveything up and sends it to the holding tank in a 1" hose. Easy, not too expensive and the girls like it cause all you have to do is push a button, no valves to position, no handles, pumps. just real easy. I also use my fresh water system instead of raw water. Keeps it a alot fresher.