It sounds like you plugged the trailer into the campground power box without turning the panel breaker off first. Always turn off the breaker on the campground panel before plugging in or unplugging. This will eliminate the sparking you saw.
Regarding the converter, with all external power disconnected, measure the voltage at the battery, then plug the trailer in and measure the battery voltage again. If the battery charging section of the converter is working, the voltage should be higher.
2014 Jayco Jay Flight 33RLDS travel trailer, 2005 Chevy 2500 Duramax diesel with 60 gallon auxilary bed mounted fuel tank, Honda EU3000 generator.