If you keep the opening smallish instead of removing the entire back of the cab, a simple 1x1 steel framework outlining the cut section would probably be more than adequate. The biggest problem you'll have is building a waterproof system between the cab and the camper. I wouldn't connect the camper and cab in any structural fashion, as it will guarantee undue stress on both items due to frame flexing in any off road situation. Possibly a gasket made from Hypalon (think inflatable zodiac) which lines the pass-thru would be best. I'll be interested to see pictures.
Spence