What "they" were telling you about is called a "cheater" box. This is a device that has two plugs on it..one plugs into 30amp and one plugs into 20amp. They used to work ok but the new codes require a GFCI protected circuit at the pedestal and these will not work on a GFCI circuit. Save your money as the cheater box allows 50amp coaches to draw more amps from an older pedestal box.
A better solution is to look at your coach's wiring and pull the A/C off that circuit and install a separate feed for it. This allows you to plug just the A/C into the 20amp receptacle at the pedestal. If you have two A/Cs just rewire one of them...this should solve your problem....Dennis
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Cheater Box