Most recent trailer I had was a 24ft 2000 Pace Shadow. I towed it 2 or 3 long distance trips in early 2000 (several hundred miles roundtrip) before installing a nose cone on it. A somewhat unscientific comparison, as terrain, headwinds, etc all affect mileage when towing something with that much frontal area.
Having said that, though, it appeared that I gained about 1mpg from the cone installation, from 10 to 11. Not as much as you may be hoping for, but diesel isn't likely to get any cheaper, if any, for a long time either. I was driving in the same speed range you mentioned...obviously we both would get better mileage if we just slowed down 5 or 10mph. 
I also noticed a definite improvement in how the trailer handled crosswinds. The nose cone generally promotes smoother airflow around the entire trailer, since you've decreased the turbulence otherwise created by the squared-off front corners without the cone.
The other way to increase mileage with an enclosed trailer is to do something about the huge amount of drag created by the squared-off *rear* of the trailer, but that would require a boattail attachment of some sort. The trucking outfit that hauls BMW's from Spartanburg, SC to my part of the country uses rigs with that sort of arrangement on the rear end of their trailers, but they can unload them in parking lots and don't have to deal with backing them up to a loading dock, which is what has prevented their general use by the trucking industry so far. Probably as much or more to be gained mileage-wise than at the front of a large trailer.
GH