Well Sandy Shores is based on the reallife Salton Sea State Recreation Area, as stated on page 448. So it's very likely to be located somewhere on the shore of the Alomo Sea
I mentioned yesterday that it could be the unknown 'trailerpark' we have seen on previous screenshots. I didn't provide any screenshots to back it up, so I tried again after seeing the discussion again.
As mentioned earlier, the red cirle could very well be the region with the train bridge, making this screenshots orientation NW. To the left of this screenshot could be Mt. Zancudo. The amount of telephone poles and palm trees (more visible in the trailer) give me a feeling it is the region in the second screenshot. The road textures in the first screenshot looks like a dirt road intersecting with a main road. So, from the moment you leave Sandy Shores at the sign, you're crossing a bigger road. I marked a possible region in the second screenshot (green) where I believe this entrance could be.
Again, it's a bit far-fetched.