First I'd upgrade that Magnetek to a Progressive Dynamics (Just the electroncs in back of the fuse/breaker panel. 4600 Series at PD)
Next: a GFCI is not current sensitive in the manner of a circuit breaker. Just as I got back from Church and shopping today power "Blinked" one of my A/C's tripped the circuit breaker as it was trying to re-start with a full head of pressure. I re-set it after it had time to bleed down a bit and all is fine.
That breaker tripped because that A/C can draw over 20 amps stalled compressor.
Circuit breakers however are like a balanced teeter totter. Perfectly balanced neither end touches down .. The GFCI measures current in both the black and white (not and neutral) legs. and so long as they are balanced.. It just sits there and observes. (passes current) but if either end "Touches down" (If the current in one let is even slightly higher than the other CLICK and shut off.
So fast you won't even have time to feel the shock
The sparking when you polug in.. Normal believe it or not.
The tripping of the GFCI. not
TUrn off all breakers inside the RV. now plug in and also if it's a duplex outlet plug in a drop lamp or other light you can see WHILE at the breaker panel.
If it does not trip. Great
Turn on first the MAIN breaker.
then one at a time turn breakers on till the light goes off.
Identify the last breaker you turned on.. Turn it back off. Reset the GFCI (light back on) and continue till you have tried EVERY breaker. the breaker(s) that turn off the light. Report what they are.
Some converters (I DO NOT KNOW IF YOURS IS AMONG THEM) will trip a GFCI if the batteries need charging.. Others (SAME QUALIFIER) if they are full. Some all the time and some never (Again Same qualifier all 4 times)
I only know for sure ONE and .. that's a Progressive Dynamics 9180 with Charge Wizard. I assume other PD's (Does not trip GFCI) but I'm only 100% sure of ONE. and that.. of course.. is about 25 feet from where I"m sitting.
Home was where I park it. but alas the.
2005 Damon Intruder 377 Alas declared a total loss
after a semi "nicked" it. Still have the radios
Kenwood TS-2000, ICOM ID-5100, ID-51A+2, ID-880 REF030C most times