The part of the book that discusses the Salton Sea and where McCandless was staying is one of my favorite parts of the book, because I frequently ride dirt bikes out there. I know exactly where he is staying. Alex is away from the lake itself, and the text says that he is in the Anza Borrego state park area. That puts him southwest of the lake.
Away from the lakeshore the land rises gently and then abruptly to form the desiccated, phantasmal badlands of Anza-Borrego. . . McCandless slept on the sand under a tarp hung from a creosote branch.
The text gets a bit more specific after Alex meets Ron Franz. Franz asks Alex where he is staying, and Alex says just past "Oh My God Hot Springs." Alex wasn't the only person staying there either. It was a combination of aging hippies and nudists. Franz didn't like that Alex was staying there, but Alex clarified again that he was out past the group.
McCandless, however, wasn’t living right at the springs; he was camped by himself another half mile out on the bajada.