You could also try posting in the San Diego Star Wars Society message board, there might be someone that lives nearby that would be willing to go along with you or provide some more exact directions to help out. http://www.sandiegostarwars.com/ - you'll have to register a new account to gain access to the forum
I went out there in May 2010. I remember it was really quiet, we were the only ones in the car park and we only saw one person out riding the dunes on a motorbike. Later a couple of border patrol quad bikes stopped by to ask what we were doing, they were OK with us when we explained we were looking at the Sail Barge filming location and hoping to find a few pieces of plywood and foam (which we did).
We even found a few metal poles sticking out of the ground by an inch or two, it seems rather than remove them some were just sawn off at the base. Perhaps they were sunk into some concrete foundations, who knows?
I'm told it gets busy in the Summer and you shouldn't really walk out there in the dunes because of the riders and drivers, but like I said, we were the only ones in the car park and it was only about half a mile / 1km from the car park to the location. As we walked we saw traces of an old broken tarmac road (under the shifting dunes) leading out to the valley which makes sense as in the old videos you can see the road and the equipment trucks that must have used it.
It was at Buttercup Valley in the Imperial Dunes, and the location is in California, whilst the cast stayed in nearby Yuma, Arizona.
How hot was it when you were there? I'm considering walking out there. How long did it take in sand dunes to walk a half mile??
Did you have the GPS coordinates because some people I've talked to have said it takes hours to find out unless you have a metal detector. How did you end up finding it?
Hi Gary Take a look at the SW Locations World Map http://www.starwarslocations.com/staticpages/index.php?page=worldmap and have a look around Yuma, AZ. It shows you the car park and the film location in the dunes. Its a short easy walk maybe 30 mins. Temperature wise it was moderately hot, worth taking a hat some sun screen and some drinking water, but not unbearable. For research watch the "Making Of" documentary that came out with ROTJ as it includes a helicopter fly-over of the sail barge set which should put everything into place when you compare it to the World Map.