While plugging into 50 amp the plug in your hand should be "Cold" that is no electricity, so any metal you hit OTHER than the contacts inside the outlet should not matter at all. I think there is another issue.
And if you plug into the park the plug the marinco connector into the RV.. Same thing the exposed metal is the safety ground, whould not harm anything .. However that is not the proper way to plug in (Whenever possible you connect COLD, that means plug cord into RV, then with park breaker OFF plug into park, then turn breaker on.
What you describe (hitting metal with a plug and causing a spark) should not be possible.
UNLESS.... You have in inverter with a bad transfer switch.
NOTE.. If you do have an inverter with a bad transfer switch... YOu have a problem that needs attention.
Recommendation.. Find the breaker on the main breaker panel that feeds said inverter and TURN IT OFF before plugging in, SOME of the coach comes li\ve you may try turning it on but this can cause issues.....
I have seen an inverter smoke when this happens.