Aahh yes, I know the type of road you were on. Curvy, narrow, steep with a few potholes thrown in. The bad news is there are lots of them out there. The good news is you shouldn't have to travel them often.
I'm pretty fanatical about planning my route before I leave on a trip. I use a road atlas (make sure it's current), but mainly I use my PC (Google & Yahoo maps etc). I seldom use the directions they provide because they can lead you astray like GPS, but the maps seem to be pretty accurate. Plus, you can use the satellite imagery to verify that a road actually exists.
When I do travel a road like you describe, I switch the tow/haul mode on to climb a hill, but usually disable and manually downshift when decending. When decending in tow/haul and you hit the brake, it will downshift whether you want it to or not, and I feel it holds the gear too long.
Since all my trips originate in the hills/mountains, I decided to add a Banks Power Pack for a little kick in the pants. Now my big turd is hot sh**. I make frequent trips south on I-79, US 19 and I-77 where there are several 5 & 6% grades and take them all at 60 - 65 mph.
Rod & Carla B.
The Hounds Kooper & Abby
2015 Sunseeker 3010; '14 Ford Focus toad