its a bit of a tricky one this. i keep marine fish (on a largish basis) and algae doesnt always need sunlight to multiply. if you have a translucent tank or a light coloured hose pipe there is potential for algae growth. - so in your case a tank in the van if translucent can get algae in it via refraction of sunlight by the inside of the van metal work!
i use RO water in my 350 litre fish tank and it has no direct sunlight near it atall (and no chance of refraction off the room walls either), yet the man made artificial light creates lots of algae infact once a year i have an algae bloom just like you get in the wild!
the question here is therefore two fold:
1. is the algae actually bad in your van tank? i.e. will it affect your pole / the clean on the windows?
2. once its in your tank can you get rid of it? now this is very difficult - the only way i have ever got rid of algae if to drain a tank (or holding tank i.e. your van tank) and leave it empty and dry for about 3 to 7 days!!!!
however IMO painting your tank black will certainly help reduce algae and may even stop it. this is another reason why many fish keepers paint the back of their tanks in dark colours (it also shows the fish up well) it helps reduce algae growth!
i may even post a picture of my fish tank now ive mentioned it!
DC