The salesman would have been referring to this 13-pin system which has superseded the 12volt N and S (supplementary) 7-pin sockets on more recent caravans and towcars.
(It's possible with suitable adaptor leads to connect a 12N+12S fitted caravan to a 13-pin fitted car, and vice versa.)
The adapter that Hacksaw Bob has suggested is the right lead you need but the long cables hang down even when you feed them over the top of the towball which you need bungee cords to suspend them to stop them falling to the floor and damaging the cables. However, I recently changed to this type of adapter which is easier to fit and does exactly the same job without incorporating the two long cables.
Until a few yrs ago the the UK used a different system than Holland/Germany. The socket(ie plug on caravan into socket on car)in your picture is the old type German Multicon WeST 13pin which can also accept a 7pin trailer plug.
These plugs/sockets have been replaced by the standard 13pin type now used in both UK & Holland so as pointed out your new car will have newer type 13pin socket & the easiest way will be to change your caravan plug to the up to date type of 13pin plug.
Is your caravan fitted with a battery that charges while you tow & do you want your fridge to operate on 12v while you tow? If so then ask your car dealer to fit fully wired 13pin socket. If you do not then it will probably cheaper to have 13pin socket partially wired just to power the road lights.
We had car problems while on vacation in Germany and had to rent a car that had a 13 pin plug (while our caravan has a 7 pin).
I appreciate all the information given me here but I learned very quickly in Germany this really wasn't a big deal at all.
I showed the VW people a photo of our caravan plug and was simply told you have a 7 pin plug and need a 13 to 7 pin adapter which is found in hardware stores and gas stations.
it would also depend on the age of your caravan as the wiring systems changed after 1997 so if your van is pre 97 your cars tow plug wiring need to comply to your van