Since none of us are there, all we can do is speculate. But the simple fact is that anything with enough current to 'melt' coax at the connector needs some seriously stiff current, probably 20a at least depending on how long this takes to fully cook if this is some sort of ground loop. IF this is the RV antenna supply feeding a dead short, there is still several amps needed to heat the cable. That would mean that the RV is wired wrong, and the power supply either isnt fused or is over rated. Sooner or later the magic smoke will appear.
As for the picture, the black ring looks like a deteriorated 'O' Ring
