The trailer owner saved a bunch of money by not having to buy an adapter for this purpose (probably $10 or $15 at your local neighborhood trailer accessory store). I bought a travel trailer recently and the dealer threw one in on the deal.
Taking a closer look at picture #3, has the trailer cord separated from it's plug? Looks like I see individual wires disappearing into the trailer. Looks different than picture #1. Strange - - -
There are 10 types of people. Those who know binary, and those who don't.