A 30A plug does not have 240V it is 120V only hence it cannot supply 240V to your RV. On your extension cord new 50A female connector connect gnd to gnd, neutral to neutral and 30A hot to BOTH 50A hots in the plug. This is how a 30/50A adapter is wired.
However consider: If the extension cord doesn't have the standard 30A female RV plug install one and buy a 30/50 adapter. Another option is buy a 50A extension cord and adapter, yes more expensive and heavy but allows for connecting to a 50A pedestal when it's to far from the RV. I occasionally run into this as there is absolutely no standard pedestal placement including even located on the passenger side.