I’ve been on site with the ones built out of a portable, like you see on school grounds. They have full flush facilities, they’ve got a separate water reservoir set up underneath or alongside that’s hooked up to a fire hydrant.

Hell, you can get hand wash stations that work on a foot pump using grey water that are pretty space-saving, even, if you don’t want to totally ditch the portajohns.

The only problem is the people who use them and leave them filthy as fuck.