I keep an old 1/2 gal juice bottle. When we arrive at a campsite, I fill the bottle being careful not to touch the spigot. I find a watering spigot or something that will not be used for drinking.
I then pour the water into the top tank of our Thetford 135, also not touching the fill opening on the pp. This lasts us about 2-3 days.
We only use the pp for pp at night, although it has come in handy traveling with no facility in sight.
Every morning, I set the pp outside the egg, disassemble, tote the bottom to the bathroom, turn the emptying tube, carefully remove the cap so I don't lose it, pour the contents down the toilet, squirt water into the emptying tube from a non drinking spigot, (usually there is one around bathrooms, staff use them for cleaning up), then pour this down the toilet, flush the toilet, reattach the cap, take the bottom back to our campsite, pour a "glug" of green stuff into the bottom section, reassemble the pp and place it back in its little cubby hole ready for the next night.
The article is interesting, but refers to the siphon toilet like you have in a home or public bathroom. Porta Pottys work differently. Here is a link to the owner's manual
Porta Potti 135/155/155SL PDF
ONE HINT: The waste compartment of a pp is sealed. This means if you drive up to the mountains, and then open the valve, you will have sea level pressure air which will whoosh out.
ALWAYS OPEN THE VALVE FOR THE FIRST TIME BEFORE USING AT EACH CAMPING SESSION WITH THE LID CLOSED AND THE BOWL EMPTY.
Don't ask me how I know this.