Hi, welcome to the forum.
Buses (and motorhomes) have two VIN. The first is the chassis VIN and the majority are GMC chassis (P30 derivatives) and start with Cxx. The second is the number designated for the completed vehicle and issued by Airstream.
So in general you use the Airstream VIN for identification of 'type' and registration, the GMC VIN however is critical for identifying the chassis when buying parts like running gear.
The GMC number gives no information on the parts Airstream added, or when they added, so don't be concerned if the GMC number is from one year and the Airstream from the next, it just means the chassis was built and held in stock until Airstream built. As the Airstream model year describes 'type' this is the model year.
As for the decoder they are on-line but here is an example.
B because it is a bus, 28 means it is 28ft long, L is the floor plan, 8 is the model year and V means it was built in Versailles, Ohio