There's no need to turn off the overdrive unless it starts to hunt--that is it keeps kicking in and out of overdrive. That constant shifting will heat it up. Use that as a guide. There's no reason to just arbitrarily keep an auto out of overdrive. The transmission downshifts due to engine torque demands. If it stays in overdrive, that means the engine demand is within it's limits. I would punch the cruise on at 60mph and turn on some Wilson Pickett or a James Lee Burke audiobook and enjoy the drive; thumbing the OD off in headwinds or grades that make it hunt.
As to the weight--that's not an issue. It's only an issue on semi's because of road damage. What is the major, major issue is the frontal area of that trailer. That 6' wide U-Hual is no problem. It has surge brakes and if you load it evenly, no problem.
Pulling a 24' long travel trailer with a Ranger is insanity. Those are 8' wide and at least 8-9' tall. A Ranger is about 5.5' wide and 6' tall. That trailer empty will kick your trucks butt just because you don't have enough truck in front of it to punch a hole in the air for the trailer. Plus the softly sprung Ranger, or even light-duty full-size, squirms around with that big lever wagging it. Even if it isn't heavy, it is going to have a lot of dynamic forces since there weight it does have is a good distance from the back of the truck. Definately use a pair of spring bar equalizers and a pair of sway controls to keep it under control if you absolutely have to do it. I think a vehicle with something around 7,000# GVWR is about the minimum for such a big trailer.