[77newell]

If there is power coming out of the main box, confirmed by actual VOM measurement, and the same wires coming into the pole boxes have none, then there is an open somewhere in between. As Guy noted, the most likely place is at a connection. Since two sites are supplied by one wire at the main, somewhere the wire splits into 2. That connection would be my first place to look.

I would expect campgrounds to run multiple sites from one wire because it substantially reduces the amount of wire that is needed to feed the sites. Depending on the campground the number of sites serviced by a single line can vary, the tradeoff being that as more sites are added to a circuit the bigger, and more expensive per foot, the wire needs to be versus running smaller wires more total feet. Where we are vulnerable is when too many sites are serviced by too small a wire and our site volt drops and heating of the wires occurs.