Never had a problem with my 2011 Ecoboost in the Cascades of Washington State, the Rockies in Montana and Southern B.C. & Alberta.
The trailer was 8000-8500#.
In general, we never had a problem going downhill while on major roads.
The only problem I've had was in the Appalachians on long,slow descents with numerous 20mph hair-pin turns and a 35mph speed limit normally. There I had to run it at very high rpms in first gear, or use a higher gear and smoke the brakes (trailer and truck) on the hair-pins.
