Los Algodones and Quenchan Indian Land
Los Algodones is a town in Mexico on the corner where Mexico, California, and Arizona meet. It has many Dentists, Optometrists, and Pharmacies where people from the US go to get those services cheaper. Most people pay to park in the lot on the US side of the border and walk. The town knows where its money is coming from, and is very tourist friendly. The sidewalks are crowded with merchants trying to sell stuff and barkers trying to get you into their businesses. There is no checking foot traffic on entry into Mexico, but the US does ask questions when you come back. The line (shaded with benches most of the way) took about an hour in the early afternoon.
I purchased a hat, two belts, and a poncho on my first trip, and got an eye exam and ordered glasses on my second. While some simple glasses can be made in hours, mine will take a week or so. Both times I ate lunch at taco stands.
The Quenchan Indian tribe operates that parking lot, an RV park just a bit further, and a Casino at the Interstate 8 junction. Many RVs boondock on their land, I stayed on just north of the freeway and was informed on the third day there is a $10/day fee for doing so, with discounts for longer periods. (Freecampsites.net incorrectly had this listed as a free camping area.) The fee includes fishing, apparently on the nearby All-American Canal. One problem with where I stayed was the frequent noisy trains.