This is from the official website. It also shows where all the LTVAs are in the area.
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https://www.blm.gov/visit/midland-ltva
"17. Self-Contained Vehicles
a. In Pilot Knob, Midland, Tamarisk, and Hot Springs LTVAs, you may camp only in self-contained camping units.
The La Posa, Imperial Dam, and Mule Mountain LTVAs are restricted to self-contained camping units, except within 500 feet of a vault or rest room.
b. Self-contained camping units must have a permanent, affixed waste water holding tank of 10-gallon minimum capacity. BLM does not consider port-a-potty systems, systems that utilize portable holding tanks, or permanent holding tanks of less than 10-gallon capacity, to be self-contained".
Not to worry. When I was at La Posa South last February (being high season), the vast majority of people in there were regular RVs and self-contained. There were a few tents and a number vans. And lots of open places to camp. I camped less than a mile in to the east of the main road, and could see 3 different toilets from my campsite, although they were a ways apart.