"Custom Camper" is an emblem only option that means the truck met or exceeded qualifying recommended options for truck intended for use as camper carriers.
Looks to me like a 69-71 GMC 2500, 8' bed and 127" wheelbase.
The best place to start is with the VIN plate confirming rosette rivets and the SPID label inside the glove box. These will tell you a lot about the truck, at least "as built".
Here are some links to decoders for the VIN and model number.
As well as manual links.
* VIN/Model Decoders
* Power Team Charts (engine/transmission/gear ratio)
* Post Your Pickup SPID Blazer SPID Suburban/Panel SPID
* RTFM ... Read The Factory Manuals... download 'em here