The plug on that cord is a regular 15 amp plug. You can plug it into any 15 amp receptacle.
The end that connects to the trailer has been modified by a previous owner but is safe to use.
Plug the 15 amp end into a receptacle and use your meter set on voltage and insert into the other end. Most likely one of the slanted slots is hot, one is neutral, and the straight slot is ground.
Then connect to the RV and see if you can duplicate the same measurements inside the RV at the breaker panel. Be careful, potential of electrocution exists when testing live circuits.
Tell us what you find.