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In theory you are reducing the air you have to move (or in this case move a second time) and you should see a gain in MPG.

In reality things arent that simple, the aerodynamics of the truck are not always improved by the camper shell and the shape may actually draw air into the gap between the truck and trailer making the MPG suffer

 The only real way to see if your particular vehicle will benifit is to get one and try it,,,

   For my money I would buy a cheap one on Craigslist (etc.) and try it out, it may prove no gain but you may like the dry somewhat secured truck bed,, if not put it back on CL and let someone else have at it ,, and just maybe you will get a mile or two per gallon out of it and be happy all the way around