I've actually got all 3 types, a hard mounted 6 gallong black tank with a flushing RV toilet in the camper, an ancient Thetford Portable, and a Luggable Loo (similar to the Pett). Each has their own place, I prefer the hard mounted in the camper over the porta-potty, mainly due to height and I don't feel like I'm going in an upside down Lego. The Lugable Loo is the best on the boat, it's a 5 gallon bucket with a latching toilet seat and lid. Problem is if you use it, it does let odors out, not something you want in the cab of the truck or in the camper all night, unless you change the bag every time, which can be very expensive. The Porta-potty is easier to keep up with, just dump it and rinse it, not a problem, unlike the hard mounted unit in my camper which requires the 20' sewer hose to empty. Just a pain to store if you don't have a storage compartment for it.
As for chemicals for the holding tanks and porta-potty, I pick up the liquid from Walmart, just read the side, if it say's Septic Safe, you can dump it anywhere. The other stuff isn't septic safe and will kill a septic system in no time, just like anti bacterial soap.
Good luck