get one of these until you fabricate that sleeping nook. the airdam I mean, not a new truck.
Enclose the bottom of the front end; most of the suspension is hanging out in the wind. preferably add a complete underbody that is smooth.
Even an NA diesel should get better than 5 mpg; fix that thing! The fuel system likely needs a *lot* of TLC to clean it up.
A turbo diesel is like a turbo gas engine; for a given *power* range, the turbocharged smaller engine is more economical for light duty use than the larger NA engine. If you already have the power you need, it really won't give you much fuel economy improvement to add a turbo. You would need to swap out the current engine for a smaller turbocharged one to see big gains from a turbocharged diesel.
How much fabrication are you willing to do to that cube?
Edit: Also, check your alignment. It probably isn't that good.