Sorry, thats not an RV plug I've ever used (Maybe its for a marine application, check at boat dealers) it slightly resembles a NEMA 11-30 intended for Three Phase. If it were mine (unless you find a matching cord) Id convert everything over to the standard RV and Campground 125 Volt 30 Amp 2 Pole 3 Wire Grounding plug and receptacle. Most campgrounds use that 30 amp 125 volt RV type outlet and the ONLY adapter you ever need is the 30 amp down to standard 20 amp NEMA 5-20P in cases where you only have standard household type 5-20R receptacles available. A twist lock type of plug and receptacle may also be in order but dont forget to unhook before driving off lol
For 30 amp 120 volt RV uses the three conductors are the Hot UnGrounDED Phase Conductor (Black) ,,,,,,,,the Neutral GrounDED Conductor (White),,,,,,,,,and The Bare/Green Equipment GroundING Conductor. The Bare/Green Equipment GroundING Conductor should have a good electrical bond to the RV's case and frame (but NOT the Neutral) for safety and shock protection.
Maybe someone who has a cord or a fitting for that particular RV or Marine plug can shed some light, its just not one Ive ever used.
Ive come across this problem with several Rv restorations. I just converted the plug assembly to a standard 30 snap lock Rv plug .(widely used in Rv parks and campground shore power pedestals). Quick racking your brain on old equipment,, Time to be safe and upgrade to avoid an unwanted short or even worse,,a fire. Simply 3 wire hook-up,, green being frame ground.
And always make sure you have a good frame body ground,,,, EVEN for cabin Batteries.
I sure agree, you cant stress that enough INSURE A GOOD FRAME GROUND. Its typically at the RV electrical panel where the incoming Green/Bare Equipment GroundING Conductor bonds to the RV frame via a copper wire out of it down to the frame somewhere.........That way if a hot wire gets shorted to the RV the Equipment GroundING Conductor provides a dedcated low resistance fault current return path back to the panel so the breaker trips clearing the fault and de energizing the circuit SO YOU DONT DIE if there was a short and you come in contact with the RV while standing perhaps on the wet earth OUCHHHHHHHHHH,,,,,,,,,,,,,,,,,,,,
John T Retired Electrical Engineer