I have a 2013 F150 w/the Ecoboost and a 7000lb travel trailer. It's not uncommon to be in 2nd gear going down steep hills.
As others said, the engine will rev really high, but they are designed for that. The 3.5l does not have as much engine braking capability as the larger V8s did.
One thing you will learn as you gain experience is to slow down at the top of the hill. If you start at 65mph at the top of a 6-10% downhill grade you will have a much more difficult time maintaining speed than if you say, slow down to 50mph at the top of the hill. Watch how the big rigs handle the large downhill grades.