Melting 30 amp plug

I have had two plugs melt on me while plugged in to a 30 amp post. This has happened over period of 8 months. Both campground might be a little suspect on the electrical side looking back. What is this caused by? Im going to purchase a surge protector before my next outing. Should I replace the plug, or buy a whole new cord. Also is there any difference to look for in purchasing a new cord.

Thanx for any help.