How common are parks with "cheater" 50amp service?

We've had a 50 amp rig for 5 years now and travel 6 months of the year. We hit a lot of parks around the country. I check each receptacle before plugging in and have never found a cheater. I have found a bad leg with no power once... got the park maintenance guy over and we found the wire had loosened from the terminal on the receptacle