Had a similar problem with my EcoB going down a steep Appalachian grade with lots of 20mph hairpin turns.
I ran it in 2nd gear but had to brake a lot for those hairpins. I tried first gear but revving up over 3500rpm sort of freaked me out. So at the bottom you could smell the brakes (and the trailer brakes) but there was no permanent damage.
We're just not used to high rpms with the EcoB.
But everybody tells me that's the only way we'll get much engine braking. So next time down that grade, I'm putting it in first and let it wind up almost to the red line. Then hit the brakes hard to drop it down. It'll scream, but that'll be better than the smell of cooked brakes.
And, your trailer brakes are probably set for emergency braking when just warm. You'll find as they heat up going downhill you can up the voltage applied without locking them up. Having the trailer brake itself properly by doing that will take some of the strain off your truck brakes.