CalebD is right, you need the pumpout fitting on deck as well as a vent line to a vent. But in most cases you should be able to plumb that straight up from the Porta potty tank to the deck fitting. Not too much snaking around and minimal hose. You also have the benefit of being able to unhook the hoses and empty the still the porta potty if you're in an area without facilities and have a full tank. Granted, that's not a fun job, but you have that option.
I did this on my boat because when we bought our boat, everything reeked, hoses, tank, everything. So I ripped it all out to let the boat breathe for a while before I install another "proper" marine head system. Our marina does not have pumpout facilities but it does have an RV dump station so emptying ours is pretty easy. We sail in some remote areas without services so I like being able to empty the tank in an outhouse or at a gas station. One last thing that made my decision easier, I bought Peggy's book and as I recall, we needed about a 15 gallon holding tank to be equivalent to a 5-6 gallon Porta potty (this factors in the amount of water needed to flush the waste down and through the hoses to get it into the tank and not to where it's just sitting in the hoses.
Hope this helps!
