I have a 1997 Dodge B3500 (one ton) cargo van that I have done a very modest "camper conversion" to. My main use for the van is towing but I added a folding seat/bed from a typical conversion van, a sink, porta potty and some storage cabinets. I have it so that I can remove the camper stuff, returning it to a normal cargo van, in just a few minutes. Again, towing and hauling stuff is my main use and having a place to sleep while traveling was my objective.
That being said, converting a cargo van is one of the most challenging conversions you can do and that is one of the reasons that they are so expensive. Every panel in a van body is made up of compound curves, up and down, front to back, everything curves. Once you get past the intersection of the cabinetry/vehicle interface the rest is downhill.
You stated that you were going to pull a small cargo trailer. Perhaps it would be better to get a larger trailer and convert part of it to a camper, ala a "toy hauler". It sure would be easier to build (everything is straight) and cheaper to boot. A cargo trailer conversion is quite popular and there is a forum devoted to them over on the tnttt. Take a look and see if it is for you.
http://www.tnttt.com/viewforum.php?f=42&sid=fb2b78700bf05be36984a04cff279356