Dispersed camping is camping in a non-designated spot on public land. In other words it's not a campground and there are no signs anywhere that say, "Camp Here" You chose a spot and camp there. They do ask that you use spots that have been used before and not create new ones, but that pertains to rock fire rings, and i never build fires.
Nearly all National Forests (as opposed to National Parks, they are different things) and BLM land allow dispersed camping. In the few times it is not allowed, I consider it their responsibility to put up a sign and tell me that. In other words, unless there is a sign that says I can't disperse camp, I assume I can.
While the National Fores almost universally allow dispersed camping, the National Parks almost universally forbid it. Death Valley NP may be the only one that allows it, I'm no certain of that. Big Bend NP has something like it, but you must get a free permit and park in designated sites.
Very, very few places have any rules about porta potties. Digging a 8-10 inch hole and pooping in it is almost allways allowed and, again, I consider it their responsibility to post a sign and tell me if it is not.
This is one of the very few times I've seen where dispersed camping is NOT alowed, and it is clearly signed to alert the public
Bob