What are you plugging into? I assume it's a campground 30A receptacle, right? If it's only a household outlet you could easily be overloading it with normal Oliver things like the water heater, fridge, battery charger, etc.
But this did happen to us once at a campground. We had a loose connection at the water heater. Don't know why it blew the 30A breaker at the pedestal and not the 20A breaker inside the trailer, but it did.
Anyway, check everything mentioned above first. If you don't find anything wrong in the shore power connections as mentioned above try this: while unplugged, turn off every breaker in the Oliver breaker box under the dinette, then try plugging in again. If you blow the pedestal again you've got a short in the cable connections or EMS. If the pedestal breaker didn't blow, turn on the 30A main Oliver breaker. If that's ok, keep turning on Oliver breakers one at a time until you find the one that's tripping the pedestal.