What is the location of the GFI that is tripping, on the house or in the RV? I wouldn't believe the dealer saying the manufacturer said you have to be plugged in to 30 amps. About the only way I believe that is because a 30A is usually not GFCI protected. You mentioned trying multiple GFI's and the problem is the same. Is there a particular reason you are only plugging into a GFI outlet, or is your whole house GFI?
Try shutting off every breaker in the trailer including the main and then plug in. If the GFI doesn't trip try turning on the breakers one at a time starting with the main until it trips.
