Mileage sounds acceptable to me. Too many variables to make a hard conclusion. I took the Prius up to Marianna to help with Hurricane Michael tree cutting last week. On the way back home I got 43mpg on the first leg and 58 on the second leg running the same 72mph. Both way out of it's normal 48-53mpg range. I agree that side wind hurts more than a head wind of the same wind speed. My F250 is more stable at 17~~mpg highway empty and 8.5mpg towing. Those little U-Haul trailers are like drag parachutes. I like that they use bias ply tires. The benefits far outweigh the slightly, possibly immeasurable reduction in fuel mileage. I have Carlisle bias ply on my boat trailer. I might put a set on our 32.5' TT.
There's a lot of OCPD up in here, lol!