Erratic images, cartoons, natural stuff.......colourful and bright........something that would catch an eye and defocus from the actual shape of the caravan. What about NCA idea. Have you seen what they have done to thier buses? I bet this task will be a treat for them. Try to meet some professor there.
Question...... what are the arrangements for powering things in the caravan......battries, generator, solar!!!
You can also go for some decent lighting arrangement on the exterior..... powered by dynmo driven through caravan's tyres.