Electrical problem..Magnatec converter?

Have you metered the batteries while connected to shore power to verify that the converter/charger is working? Is the converter on the "Receptacle" circuit that you say trips the main breaker. Sounds like you need to meter each 110v circuit at the main RV panel to see which one is the problem. Also, besides a short a missing or loose connection can cause a GFI to trip. If you don't already know, trip each breaker on your RV service panel and determine what each one powers. Since the "Receptacle" breaker throws the main RV breaker then by process of elimination you should be able to figure out what is on that circuit. Then check each item. Don't forget your refer and water heater if either or both have electric element heaters, they will cause a GFI to trip if they are bad. Unplug your refer and see if that makes a difference. Also the water heater if it has an electric heater element.

John