I've found that the combo of a 30A RV cord, 30A to 15A adapter and extension cord is a BAD combination. many of the 30A/15A adapters have very weak contacts on the 30A side which will loose tension against the 30A blades pretty easy. = overheating= weaker contacts= trouble. And the heat affects not only the adapter, but the nice copper high heat conductivity to the 30A plug assy. Not a good combo.
If I'm going from a 30A plug to extension cord I use one of the short "dogbones" that has a real 30A receptical on one end and a stout 15A blade on the other end with about 12" of wire in between