So it was the twist lock power cable connector end which connects to the coach that melted, not the shore power pedistal plug connector end? When any cable electrical connector gets hot from high load currents and perhaps less than ideal (meaning resistive) connections between mating electrical surfaces, you usually start getting metal surface oxidation and extra heat generated. Over time and under continuous high load currents the oxidation gets worse and can result in the electrical connector surfaces getting even hotter; making more oxidation (more resistance to current flow) etc. So it pays to regularly inspect the connections at the cord plug/receptacle ends as well as on the connections on the outside of the coach for signs of discolored electrical connection surfaces and overheating. Glad that no fires happened in your situation.
By the way, this problem can happen on any electrical connector/connection, not just twist locks. Look how many folks have had burned up transfer switch wires from poorly connected marretted wiring splices.