You can test it yourself with your multimeter & a wiring diagram for the sockets(just google '12n/12s wiring') On 12n socket touch neg. probe of meter onto socket earth pole the check each pole for power at the appropriate point ie brake lights & indicators on/off & rear lights on all the time. Thats enough to make your lights work.
If you test 12s socket remember pole 2 live all times, pole 6 live engine running only.
Ideally you want to check it on your own van, for two reasons. Firstly you can check your van lights are working properly. Secondly, if you have a second hand van, you may encounter the same problem I had with a second hand trailer.
I bought the trailer and the seller told me that it had all been rewired. As I didn't have a towbar, the seller kindly delivered it for me. I watched him reverse it onto my drive and in doing confirmed the lights were all working.
When I got my tow bar fitted and checked, I got home and hitched up to the trailer. Oh dear. Everything was connected up backwards. Brake lights coming on instead of indicators, everything was wrong. Turned out the clown who had rewired it must have also wired the previous owners towbar as he was obviously either colourblind or retarded. Not one wire had been connected correctly at any of the lights, despite all being in the correct place in the plug.
iam ofton told that the lights are working on the van , they are until you add a diffrent tow car
one point , on the s grey plug , you want to load test the fridge circuit , most testers only show the fridge circuit is working , sometimes some cars (newer ones ) strugle to cope with the load a fridge puts on the relays and fuses