This is also why, in an earlier post, I said that maybe this box van just plain can't go much faster than 55 MPH.
I could see that happening, especially with a head wind. My F250 towing that not too tall but very long trailer could barely do 55 at times with head winds. Course it has 3.54 gears, but still.
My motorhome is more like his box van, but being turbo, it's a bit better. Even so, with a good head wind, it slows down a lot despite some slight aerodynamic features of it's pointy tipped cabover. Seems like I got slowed down to about 55 one day when I was heading out here to Arizona from Texas back in November. Of course, I was babying it somewhat due to EGTs. It's like climbing a long grade basically. That one day east of Las Cruces, NM, I had a head wind and a steep grade. I was down to about 20-25mph, manually shifted to first gear, for the last bit of the hill, due to having to balance the coolant temp and EGT so they were both in the safe range.