I also use Fenwicks overwintering liquid. I just use a garden sprayer on a very fine mist, this avoids runoff, and you can spray into all the little crevices where algae would nomally acumalate. Note what it says on the container about using a slightly extra strong solution to clean it off. I again use a garden sprayer doing a part of the caravan at a time. Comes up like a new pin.I also have a small quantity of 50/50 solution in a small hand sprayer for stubborn marks. If I do use the van during the winter I do not bother to wash it before or after.
Good luck.
JTS.