Well.. I used to have one for trailers (Made by the NATB, national auto theft brueau)
You might check with the local Motor Carrier officers (Diesel smokies) as they are very likely to have a NATB trailer book on hand
One thing though.. There are very solid standards for the construction of a VIN on power units (Cars, Trucks, Tractors) but as I recall not so solid on trailers
Since I think 1980, when they went to the long vin, for most vehicles the first character is country of origin (Where final assembly is done) US is 1, 4 and 5, Canada is 2, Mexico is 3, Other conteries are usually letters. W for example is West Germany
The next character or two is the company
So WV is... VW (West German Volkswagen)
Then you get into plant codes, body styles, model years, a check sum and finally the actual serial number of the vehicle (last six)
At one time I was good enough to construct a valid Chevy VIN (I will admit it was by accident.. We (Myself and an FBI agent) were testing a computer program that was supposed to help us figure out the proper VIN on a car where it had been altered.... This program today has been ported over to Windows as "Vin Assistant" on police department machines)
NOTE: a police dispatcher may be able to run VIN-Assistant on your vin for you. It is included with many police dispatch/communications packages.
As far as I know the police version of the program is more complete than the civilian Vin-Decoders out there and is not one you can access yourself.