Thinking about the parking light functionality, I just went out and tested this.
Everything switched off, then move turn indicator stalk up (right turn) - this switches on the van and trailer right hand side lights (as parking lights), as expected.
Now, repeat above, but with indicator stalk down (left turn). - this switches on the van left hand side lights (as expected), but also the trailer right hand side light (unexpected, assuming trailer lights are designed to mimic that of the vehicle).
Everything switched off, then move turn indicator stalk up (right turn) - this switches on the van and trailer right hand side lights (as parking lights), as expected.
Now, repeat above, but with indicator stalk down (left turn). - this switches on the van left hand side lights (as expected), but also the trailer right hand side light (unexpected, assuming trailer lights are designed to mimic that of the vehicle).