This is a typical camp ground pedestal for 50A 30A and 20A connections.

This is a typical wiring diagram of the 50A-30A-20A pedestal

This is test procedure to determine if the 30AMP PLUG is wired correctly..

Hope this helps...
There are a couple of other issues that may come up.
Is this a PULL OUT CABLE with the 30AMP PLUG mounted on the end of the 25-foot cord. The problem here is you must make sure the MALE PLUG will fit inside the door of this setup when you push it back into the trailer body for storage.. Some MALE PLUG HEADS are too large to fit inside the lid shown here

If this is an MALE/FEMALE 25-foot extension cord that connects to the side of the trailer with a FEMALE PLUG similar to this photo then you really won't have to worry about the size the MALE PLUG you are mounting to the end of the RV30AMP extension cord. Also be aware you can purchase a RV30AMP MALE FEMALE extension cord already made up from most any RV PARTS STORE...

ALL PHOTOS FROM GOOGLE IMAGES
If you are unsure how to wire the plug back on the end of the 25-foot extension cord I would seek professional help. Wiring it wrong could damage internal electrical items inside the trailer.
Remember what you wire up you will be responsible for anything you burn up. The licensed electrical folks are all bonded to pay for things they burn up by not connecting up correctly..
Roy Ken