i towed with a psd for 13 years without an exhaust brake. Been down 7% without a problem one.
All of it was interstate, but was a piece of cake. Watch for the curve warning signs and get slowed down before entering.
BTW that old saying of down in the same gear as up never applies to me in the smokies on the interstate. I used to have to grind up the long grades at 45 mph in second. If you went 45 down the long straight grades where you could see the bottom, you would likely be a road hazard on a busy day. Just saying, one rule does not apply to all situations in this case.
How steep and where is the dictator.