Yes, there are remote camping places throughout the area between Julian, Thermal, Salton Sea and Morongo Valley. But they were apparently sighted at an actual campground in Thermal. If they had abandoned their equipment there, I think LE would treat that as reason to change the status to involuntary missing, so I doubt that happened.
It's really uncommon to take all of one's equipment out of the vehicle for an overnight tent camp (over even for a couple of days). Things like the big jug of water that all desert campers need to take...stay in the vehicle (usually). Food storage is usually the vehicle, not the tent. Most people don't leave their cook stoves in camp while they're out sightseeing, either.
As for dispersed camping, well, that's a possibility too. There's BLM land nearby. I don't think they allow dispersed camping in Anza-Borrego State Park, though. Dispersed camping is when a person finds their own campsite - rules vary. In Death Valley you have to still park on the shoulder of the road and then camp so many feet away from the road. In other areas, you can actually drive your vehicle on forest roads and camp where ever you like. I doubt that they would have camped some place where they couldn't keep an eye on the truck. If he's at all familiar with the area, he'd want to keep an eye on his truck, I'd think.
IMO.