There are some places where one is required to pay a small fee to stay, with no real amenities provided. We saw those in Baja especially, open spaces where visitors might like to stop, locals would come and collect.
On Mainland, one was near the Presa Cajon de Pena, near the city of Tomatlan, where fishing for bass was good, the military macaws could be found; also, duck hunting was encouraged to keep them out of the rice growing area nearby. For instance, the Bay of Tenacatita was always a destination for us and, although it closed to the public in August of 2010, (at gunpoint, by state police) it just may be open again, according to recent reports. We did seven winters there, paid about 5 dollars a day to a Mexican artist from a nearby village who attended the beach almost everyday and filled an important position that kept us very happy and secure. We payed almost the same to have water delivered. There was no electricity on that part of the beach, generators were not used, a rule enforced by opinion of the majority of the campers. Everyone had solar or else moved on after a couple of days. A laundry man collected weekly, brought it back the second day after; the beer truck, Sol/ Dos Equis, came to the beach twice a week; propane got delivered to the beach about twice a season, for us it was used exclusivly by the refrigerator. Small propane bottles were filled at Cihautlan, about 50 kms south. Supplies bought in Melaque, 40 km south. Cooking was done outdoors by most, its heat was the problem; we showered outside in a shower enclosure, the water heated by the sun; black water was blue-boyed by vehicle to the back country; swam daily around the coral beach, bought from visiting fruit and vegetable vendors; bought warm, freshly baked items from the bakery lady who often brought them to the beach. Dined on tables set out on the dirt streets in El Rebelsito, ate periodically in the about eighteen palapa restaurants on the main beach (no longer there of course, bulldozed).
We once stayed in a Walmart in the city of Colima, and they seemed so happy to see us we stayed two days to take in the city and outlying area. Other than a couple of stays, maybe of two weeks total were at the now-closed Negrita Bay, on the Bahia Chemela just south of Perula, we always payed to stay in a park while we had time; maybe 2 or 3 Pemex stations a season while we were in a hurry. We had great times after staying on the beach when driving into the cooler, higher areas, Villa Corona, Patzcuaro were favourits. Never stayed in a restaurant's lot, although once in a Hotel's, in Perula. There was a rate for RV's.