Disconnect from city power and open up that outlet. You will most likely see a burn spot where the arc was. It may just be a loose connection of one of the screws on the receptacle or one of the wires close to the trailer body.
If you do not see a burn mark it may be within the receptacle, not likely but maybe, if so just replace the receptacle. The prongs inside may have relaxed to the point of sparking because of a bad connection.
This is a straight forward job, that you can do, as long as you have pulled the plug from shore power
WBCCI 12156 AIR 3144 WACHUNG TAC NJ6
2004 Excursion 4x4
1991 34 ft. Excella +220,000 miles, new laminated flooring, new upholstery, new 3200 lbs axles