Progressive EMS 30
I would bet the site pedestals are feed with aluminum wiring and those connections at the pedestals have not been checked or tighten since installed. The pedestals typically have a feed/ loop bar that allows the pedestals to be daisy chained. If somewhere in the daisy chain the neutral lug is not tight it could offer enough resistance to allow the voltage to rise durning low loads. If the two current campers are only running electric heating after midnight that would present the low constant load.
Durning a higher load the current is enough to overcome the resistance of the pour connection.
It very possible it could just be the connection in that one pedestal to the feed/ loop bar.
One could investigate the problem using a voltage meter when the problem occurs. Start at the one pedestal known to have the problem and check the voltage at one of the outlets. Then check each adjacent pedestal, and working outward to the ends of the line of sites. If you know the direction of the power feed you can just work backwards.
Checking the voltage, at the time of the problem, at an outlet in the park's facilities building would tell you if it's a common utility voltage problem, as the sites are typically a different service from the buildings.
It is possible for utilities to have power spikes, even up to 132V, but its extremely rare for them to occur consistently each night and/or for 6 hours! They are typically random and very short in duration < 1 minute; if not for a few seconds.
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