Here is a quick little tip for repairing a power or tow cable where the outside jacket is slowly pulling away from the plug.
A fellow full-time RVer named Greg clued me into this repair. First, you heat the cord either by leaving it in the sun or with a heat gun. Next, add some brown Gorilla Glue or another similar adhesive (I used G/Flex Epoxy) to the end near the connector.
Then slowly pull on the cord, working the cover toward the plug end. Eventually, enough cord material will shift forward and back into the plug. The glue or epoxy will seal it in place, preventing the problem from happening again.