A conversion van is a small RV and is a cross between a van and a microbus. It is basically a big van which can be equipped with television sets, video game consoles, hot or cold food storage, and many other things to take the place of conversation while driving. Some are used primarily as cargo vans, others for transportation of people. Passenger conversion vans are often seen with tacky "custom van art" on the side and have huge runners that come down to about 4 inches above the road.
Body lengths range from roughly 107-127 inches, according to several makes, including Dodge. People either seem to buy a used one and make it look like a giant monstrosity, or go for the new yuppified ones that cost in the range of 30-50 thousand dollars with some bells and whistles and a nice roof storage area. Full sized vans cost about 25 thousand dollars before conversion, and size does not particularly effect price because it is harder to make things smaller.
For more info, see: Vanogan and the following webpage
A thanks to cbustapeck for some suggestions.