I don't have the mains charger or indeed a mains supply in my van. Just the 12v.
I brought the digital camera with me but forgot the transformer charger.
I guess it is just a simple dc converter anyway?
No.
The root issue here is that in colloquial language, we use the word “charger” in a very sloppy way to mean several distinct things.
One meaning is a circuit that actually sets, monitors, and adjusts the charge current and voltage to a battery and decides when to terminate the charge. It knows how to properly charge that type of battery. A camera or power tool battery charger into which the battery is inserted to charge is a typical example of this as a dedicated unit.
(Very rarely, an actual bona fide battery charging circuit will be provided in “wall wart” format. These are largely extinct, mercifully, because they cannot charge any of today’s popular battery technologies properly, meaning safely and gently.)
Note that many types of devices, like phones and laptops, always have the actual charging circuit integrated in the device itself. If a battery can charge inside the device, then the actual charger circuit is built into the device. This is the most common approach nowadays.
Accordingly, the other major colloquial meaning of “charger” is actually just a power supply for a device that contains a built-in charger circuit. When you connect your phone’s or laptop’s “charger” to it, you are connecting a simple power supply. This power supply knows nothing of the battery inside the device.
So the upshot is this: your camera battery charger is an actual battery charger, while your laptop and phone “chargers” are simple power supplies supplying power to the actual battery chargers inside the phone and laptop.