SHiner,
Issue is, MANY of us still use these truck in commercial settings. Out of the 500,000+ miles I have towing trailers, less than 100K of that is an rv. The rest is what I would call local delivery commercial.
Hence why I would choose for this part of the test, the typical min speed on interstates.
Then another for folks. Back in 92 when I bought an rv, there was a ford poster on the wall. Max trailer spec at the time was 10K for a 8 lug 250/350. That was with a trailer that had no more than 80 sw ft of frontal area. it ws reduced 2500 lbs if you were between 81 and 100, another 2500 for 101-120 to 5000 lbs. and trailers over 120 sqft was NOT recomended you tow with your F series trucks. Some similar deducts were there for the Ranger and Aerostar vans with the 4.0 V6.
Marty