I have found that in Chile you have to differentiate in Spanish when you are looking for camping as two different types exist.

Hacer camping - pay to camp.. with hot water, bathrooms, separate camping areas
acampar - free to camp.. actual camping (in my book)

There are a lot of places that have signs that advertise Camping, these are all obviously pay to camp places.

I did the route from Santiago to Antofagasta a couple months ago (I work in Antofagasta now, if you come through give me a holler). I did it in 3 days about 550 km a day. I camped one night in Guanaceros, I wouldn't suggest it, in a pay for camping spot because I started looking for a spot to late in the afternoon and didn't plan correctly. The other night I camped on the beach in Bahia Inglesa (about 2/3 distance between Santiago and Antofagasta). I would highly suggest this if it fits into your route. Beautiful beach town and if you're coming through later in the year there is supposedly all kind of parties and action going on here. I guess they are lacking a couple thousand beds here in the summer time so they allow everyone to camp on the beach for free. I was talking to a guy I met on the beach about it who runs a kite surfing spot there, sounds like a pretty legit place to visit during the summer months!

Good luck with the trip!!

MLM