That doesn't sound so bad.
I've towed up and down long gravel roads over 25%, with many switchbacks. At that point, removing WD is a must for going up. But going down it down helps the front tires grip. Gearing down of course is a must, and sometimes riding the trailer brakes is useful, such as during a sharp turn.
If I'm on a really steep grade, I'll put on the parking brake for some extra drag. If I'm on REALLY steep (over 50%) I'll put the transmission in the opposite gear (i.e. drive for backing down) which lets the torque converter absorb most of the braking energy. It actually works great, and is a similar principle to the transmission retarder used on big buses. However, I'm not sure how well other transmission would hold up to this. Seem to not bother mine.
