Usually vans got lighting in the back cabin, which is operated from the starter battery. Mine was equipped with a 12V 10W incandescent festoon bulb. 10W at 12V means that there is a current of almost 1A (0.83A) going through the light. Although this is acceptable for sort time use, in my book such an energy consumption is somewhat high for longer evenings.
Modern days know LED lighting, which is about 90% more efficient than incandescent lighting. However, the electronics of some LED light sources cannot cope with varying power, have the light source failing.
There are also some cleverly designed electronics which are tolerant to voltages between 8 and 30V.
The local marine equipment store offers products of the Swedish make Båtsystem.
My light source of choice was the 94Spool1. Power rating for this source 0.7W (0.66W according to the packaging), resulting in a current of 0.058A (0.055A). Such a low current encourages of using the van's lighting all night long.
The only "problem" with this particular light source is, that it's got only one side that emits light, so orientating the source correctly during installation is important.
