Authorities permit tent camping and RV parking at some of the Bahías de Huatulco [1]. Self-contained RV parking or tenting is allowed in the big Bahía Tangolunda [2] public beach access lot for about $2 per person per day. Although the beach is beautiful and the partly palm-shaded lot is pleasant enough, facilities are limited to toilets and cold-water showers. Get there along the road along the beach to Tangolunda. Turn right at the signed Playa Pública public access road bordering the western edge of the golf course.
Other Huatulco beaches, such as Playa El Maguey, may be available for RV or tent camping. For more information, visit the Huatulco National Park office (adjacent, beach side, to the boat harbor, Santa Cruz de Huatulco, tel. 958/587-0446, reservations booth 8 a.m.–noon daily, administration office 9 a.m.–2 p.m. and 4–6 p.m. Mon.–Fri.), both of which issue camping permits (permisos).
Links:
[1] http://www.moon.com/destinations/oaxaca/pacific-resorts/bahias-de-huatulco
[2] http://www.moon.com/destinations/oaxaca/pacific-resorts/bahias-de-huatulco/exploring-bays-huatulco/east-side-bays