those 30 amp twist locks are terrible
they often make poor contact which causes arcing which caused the problem you have. Make triple sure you have a solid contact EVERY time you plug in.
I doubt marinco will give you a new cord. operator error or a bad plug on the other end can easily be blamed. Please let us know if you succeed in getting a free replacement
this happens ALL the time, ask around the docks to see everybody's plugs, you'll see plenty of burned connectors