I think if you check the 2005 Viedma meeting info you'll find more details, including GPS coord's for the campground.
Except, I think Grant may have just updated the meetings page because the links to prior year events are gone.
Someone with more time could do a search perhaps.
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ah...its back - here's from the 2005 page:
http://www.horizonsunlimited.com/mee...entina2005.php
GPS co-ordinates: 41º 03.167' W 62º 49.805' - this is for Los Trentinos
Note that your map may say "Balneario Massini" for those coordinates. Whatever, it's at the mouth of the Rio Negro.
Viedma is right on the main highway heading south from Buenos Aires. Hard to miss! You shouldn't have a problem finding the campsite once you get there, just ask for the "campground by the river". Phone Oscar for information if you have trouble. 02920-424096.
(don't know that the above phone # is valid or not - from 2005)