What you want of your porta potti is to end up with a nice slurry to empty that doesn't clog or catch on the potti internals.
In most cases the ordinary paper you use at home is perfect.
Grab a glass jam jar and 3/4 fill with water. Add a couple of squares of loo paper, pop on the lid, and shake for 5 seconds. If your paper breaks up into mushy little bits, all is good.
Just remember that number ones are sterile and without any number twos in the potti the system can build a powerful ammonia odour regardless of chemical used, so use it for both.
Yes, yes, I know, but it was the simplest way to explain to a newby that urine is never-the-less quite low on bacteria, even to the point that it is preferable to p!ss on a dirty wound to clean it than to use water from a muddy puddle.
Without an appropriately large amount of natural bacteria in the system the chemicals don't work.
If you only use it for number ones, portable toilets become very stinky very quickly.
If you use it for one and twos you can go 3-4 days on a chemical dose, without bad smells, assuming your tank is large enough.
This topic is likely to spread fast from paper type to pee or not to pee, best, cheapest, enviro friendly chemical to use, SOG units etc.
probably the best response to this topic would be from Motherhen if she is around. Or to use search as a lot of good advise in previous posts.