A voltage higher than 125/ leg is indicative of an open neutral, even if your plug in tester shows neutral good.
Camp wiring layout is done in 2 ways, 'spider' or 'loop'.
In spider, each site is wired individually to the distribution panel, master breakered there for the amperage the wire to that site is rated for. A 'cheater' will not give you more amps in this layout.
In loop cabling, the master breaker is sized for the cable that serves multiple sites. vis a vis, and in then out to the next. In either, if the service pedestal is equipped with a gfci outlet, an amperage increaser device will not work
Voltage is dependent on the utility, length and size cabling to site, amperage draw at that location.
our Utilitiiy providers are permitted a 10% variable in voltage transmission, with 120v the basis. Thus, plus or minus 5% = 126v max., 114v min. at the distribution metered point.