JAM, I was extremely disappointed with the Imperial Dam LTVA when I stopped there. Everyone was so mobbed around the 2 toilets near the entrance station, I didn't even have room to park my van where it wasn't within a few feet of someone else. Poo. I didn't even stay. If you have a non-self-contained van, then you have to park within 500' of a toilet. If your rig is self-contained, then you can camp 1/2 mile or more away from the mob and have a little elbow space.
This is one problem with the LTVAs. Tons of big rig RVs park close to the toilets so they can avoid having to dump their black water tanks as often, which no one likes to do.
The following comments refer to La Posa:
As hinted by HDR, the big rigs, usually Class-As and 5th Wheelers use a trick to get small rigs like vans and tent campers to vacate. The rule says it's legal to camp within ... 15' ... of another rig, so these guys will come in, park close, and run their generators all night long to "persuade" the small guys to move. Two Class-As tried that with me last winter in La Posa, but I said to hell with them, and stayed until "they" moved.
The other trick the big rigs use is for a group of them to space themselves out so they have a large area that they cover, 100'-150' between rigs, but they're close enough that it dissuades small rigs from getting in between.