I'm certainly far from an expert, but I can speak from experience just this past weekend, when we crossed over two mountain ranges with 7º and 10º grades respectfully, both having only a single lane each direction. Having roughly 8 years of small camper (17', 23', 25') towing experience and a relatively small amount of experience with towing on steep grades I can tell you on our trip to the campground this past week I had white knuckles, but after making some adjustments on our return trip home, it was soooo much easier and less stressful.
I have a '05 Tahoe that has less than stunning brakes even without a trailer (IMHO), and a '75 25' Tradewind which comes in at around ~5,000-5,500 lbs loaded, and has new axles/brakes.
On our trip out, I had the Tahoe in tow mode which shifts slightly different and prevents OD from engaging. On the descents, I controlled our speed solely with the brakes which means I pretty much rode them the entire time. I tried to leave off the brakes as much as possible, to limit the brake fade, but the grade didn't allow for much time off the brakes. By the time I got to the bottom of the 10º grade, my Tahoe's dash was saying "Service Stability", and I had a VW Jetta about 6" off my bumper the entire way down, not fun. I had to pull over and shutoff the engine just to clear the dash error.
Anyhow, on the way home, I manually down shifted to second gear, the transmission walked us down the grade at around 35-40mph, with limited braking required. It was an amazing difference.
I would recommend (as other have already) toying with manually downshifting to a gear (second or third) that yields roughly the speed you're after, which will limit the amount of braking required, and make for a much more stable decent.
Eric, Stacey, Easton, Annabelle, Gretzky (boxer), & George Bailey (basset hound)
1975 Tradewind - Family owned and operated since 1975
2005 Chevy Tahoe Z71