[aerohead]

John,I worked a couple years with a local moving company and the largest unit,a Ford truck-based moving van with big gasoline V-8 got over 8-mpg.Only mod was a 'Nosecone' recycled from a local wrecking lot.
I agree with the others that somethings amiss with the diesel.
A local resident owns a 'Vixen' motorhome and has reported 30-mpg with the 5-cyl diesel 5-speed( maybe 6-spd).It's probably less frontal area than your rig,certainly a lower Cd.
80,000-lb 10-wheel semis have hit 10-mpg.
20-mpg might be doable but you'd have to do 'everything' and something like a modern Sprinter might end up costing less,if those had enough interior volume.