There can be a lot for "Branding Opportunities" with your traveling Billboard....
I am doing a presentation on just this similar topic (and a bit more) coming up in April at the Experience show....don't know if you could make it....
But, think about colors that might fit your brand (branding),
A bit of theme....don't get too busy, but also you want to make everything as readable as possible from a distance.
I am just going to put up a pic of my vans, but keep in mind, there are many ways to be unique and memorable...and really use your "Traveling Billboard" to your advantage.
It is difficult to just explain it all in a post, but I would suggest take a look at vans and signs even outside our industry for things that are memorable to you.
Big Blocks and Big Blocks of color are some quick thoughts that I always suggest to look for ....
I am sure others will really chime in here too....