Porta-potti, Cassette, Black water tank, even relying on public restrooms... each choice has it's drawbacks and advantages.
The one over which you have the least control is the public option.
The porta-potti is most versatile, the cassette's biggest advantage is that it doubles as a blue-boy. The biggest disadvantage to a blackwater tank is the waiting in line, but if you have a greywater tank, that issue doesn't count.
Any of the first three can be managed and maintained easily and cleanly with a little practice, and the convenience, many times, far outweighs the work.
Overall, the advantage of the blackwater tank option is that all waste handling is done outside the trailer.