The downside to just letting go outside the door is the stench that will follow, you can't deny that peeing on the ground in the same spot after a few days isn't absolutely offensive.
the port a potty is the same way I'm thinking but it would be a regular toilet with a 20 gallon holding tank on wheels just outside on the other side of the wall. as long as I can elevate it higher than the inlet of the tank it should be good.
I'd still have to empty it in a hole somewhere though, my septic tank is actually above grade with a mounded leech field.
I got a while before I have to get it planned out perfect, but I'm sure it will all work out good.