Steve, the only difference between
a permanently installed portapotty fitted for pumpout and a holding tank is the location of the tank. The portapotty's tank is under the bowl...it doesn't take up valuable storage space anywhere else. It's vented the same as any other holding tank, the odor control issues are no different, and the 5-6 gallon models hold about as many flushes as a 12-15 gallon tank connected to a manual marine toilet. Dilution may be 90% of the solution when it comes to waste treatment (purification), but it makes no difference when it comes to odor control. Portable models do need to be dumped frequently, but only because they're sealed--anaerobic. And they don't hold enough to be worth installing for pumpout.So while your points are valid when it comes to portable portapotties, permanently installed portapotties are really no different from any other holding tank.