Too high does not explain what the RPM actually are when climbing hills.
TOW HAUL can be used all time if needed to keep the transmission from shifting in and out so much such in foothills where it downshifts and upshifts with each grade. It will not hurt the transmission.
The transmission will help to brake speed downhill in the mountains and if the RPM get too high 4,000-5,000 for a length of time them brake hard to slow the vehicle and drop the RPMs. It may actually shift up a gear allowing speed to increase. If needed to shift down hit the brakes a couple times to check the speed and the transmission will downshift again.
4,000-5,000 RPM will not hurt the vehicle for short periods of time, like climbing steep grades or descending steep grades. The V-10 develops its max HP at 4,800 RPM.