In season 8 Episode #30 (Goober's Contest)there is a scene where Floyd is in his car at the gas pumps talking to Goober. In the background you can see the end of a quonset hut with white gravels in front of it. This is part of the set for Gomer Pyle USMC, which was located on the same property. There are some vehicles parked around it but you'll definitely recognize the buildings.

There is also a shot of people lining up in their cars to get to the gas pumps and one of Floyd driving away. They have to pass through a large gate in a pretty tall fence. I'm guessing this was a fence for the studio property since the gas station was on a dirt road on the edge of town. Unless the town of Mayberry was fenced in.