17 Express 2.8 Diesel

I know there are very few of these vans out there, but do any of you find the transmission holding low gears for too long? The problem seems to be worst at about 30 to 35 mph and then highway speeds. The van will run about 2000 RPM for extended times at 30 to 35. If you accelerate to 37 MPH it will upshift ( rpms drop to about 1400) and then you can slow back down and there is no trouble maintaining speed. A similar thing happens at highway speed, the trans stays in 7th gear ( about 2100 rpm) until 67 mph and then finally shifts to top gear (about 1500 RPM at 70mph) and has no trouble maintaining speed. This appears to be really hurting mileage, since the readout shows about 27.2 MPG at a steady 60mph, but goes up to 30.3 MPG at 70 mph. WE are running about 7500 lbs GVW and tow/haul mode is not engaged. At 1500 rpm this amazing little engine is already pumping out 100 HP. Yes, I'm greedy... getting this kind of mileage is amazing, but it looks like the vehicle could get about 10% more if the transmission would up shift earlier at low load. I cannot get it to upshift even in manual mode.