These are the general guide lines that I came up with:
Empty:
55 MPH : 1500 RPM
60 MPH : 1800 RPM
70 MPH : 2000-2200 RPM (depending on the size, height and weight of the vehicle, head wind, etc)
towing:
55 MPH : 1800-2000 RPM
60 MPH : 2000-2200 RPM
70 MPH : up to 2500 RPM (assuming it will even go that fast)
Turbocharged IDI empty:
55 MPH : 1400 RPM
60 MPH : 1500 RPM
70 MPH : 1600 RPM
Turbocharged IDI towing:
55 MPH : 1600 RPM
60 MPH : 1800 RPM
70 MPH : 2000-2500 RPM (again, depending on how much load)
The overall happy medium cruising RPM is about 2000. This has enough power to spare to for some extra load, but also is low enough RPM to be fairly economical.
I geared my truck to have 2 top gears instead of just one. At 70 MPH, I can pick between 2250 RPM for towing, or 1650 for cruising empty. With the topper off the truck I was getting consistently 20 MPG or better last summer. The high fuel prices may have also had a positive effect on my driving style
Personal best was 22.5 MPG averaging between 65-72 MPH. I base my calculations on the same highway going 100 miles south then the same route back home to get a good average that accounts for slopes or prevailing winds.