These trucks are known as Dekotora [デコトラ] which is an abbreviation for "Decoration Truck." These trucks are extravagantly painted with artworks of many kind but I still have yet to come across one with moe characters on it.
Apart from the artwork, many Dekotora are also also covered with lighting and customized with metal bits n pieces - I guess the owner of the truck wants to make a point of "don mess wid me."
You can see these from time to time on the highway but seldom in the city .
If you like the look of these trucks but dont have space for one - you can always try out the Dekotora game below.First image from Isuqe's Flickr pool and the rest from Ztath.