RE: Chemicals for the portable toilet
I've used a couple and settled on the T-5 concentrate, a liquid.  One big
reason is that it comes in a small bottle that is set up to measure the
right amount for you.  I picked up a larger bottle and refill the small
bottle.  I couldn't find the liquid version online for an example, I get
mine at Evergreen RV in North Seattle. BTW, the bottle says "100%
Biodegradable".  Doesn't that mean it rots?

As for dumping or where to dump, I've used my SaniPottie for about 15 years
and have dumped it in everything from my toilet at home to sanicans at
festival sites to pit or vault toilets in camp grounds.  When I first
started using it I asked both a forest ranger and a campground and the
owners at a RV park if there was any potential problem to their systems
dumping, with the chemicals that had been added, and they no problem at
all.  The guy in the RV park actually said "Hell no, we get tons of those
chemicals into our septic system all the time from the big RVs, the little
bit you are adding is no different".