My 2001 F-53...when in overdrive, climbing a hill, if it's steep enough, the overdrive will cut itself out at some point....you can listen to the engine starting to "lug" or the rpms slowing down, and the motorhome starts slowing down... then at that point overdrive leaves and....y'gotta step on the gas pedal.
My complaint, at this point, is that the dash a/c cuts out due to a very small vacuum tank. On a hot day, an inconvienience. But as soon as the load gets back to "normal", the a/c come back on.
As far as TINSTARs' comment (can't find any place to repair it) a Ford Truck dealer can't do it? Something else going on here, IMO.......