Not sure what the grades are but I would manually Gear down in both cases. Good idea to follow the posted speed for trucks. Better to be too slow then too fast on the decent. Get yourself a copy mountain road directory (not sure if that's the correct name or not). It will give you grade percentages at each mile post marker. Don't ride your brakes, if you gain too much speed use you brakes hard to slow and then let off the brake pedal to cool the brakes repeat as necessary.

Once you have completed a couple of accents and descents it will get easier. You'll get the feel for you vehicle. Again just take it slow, let others go out around you don't let them pressure you into going faster than you want.

Never ever ride your brakes on a down grade. Use short stiff braking to bring your speed down, then off the brake and let momentum bring your speed back up, then repeat. If you ride the brakes they will just heat up and become useless. Not a position you want to be in.
Basically you will be going slow, fast, slow, fast and so on. Always start your descent slow. Use lower gears to take advantage of engine braking. If you have a Tow/Haul mode use it. If you take your time there nothing to it.

The Mountain Directory is now available as an iPhone/iPad and Android app. Very useful info.

2014 Ford F-350 4x4 SRW 6.7 power stroke (3.55 rear axle ratio) ... 3,100 RPMs in 2nd gear @ 40 MPH on a 6% downhill grade (Eisenhower Tunnel west bound) with a 15,000 lbs 5th wheel in tow. Let it gradually rev up to 3,500 rpm and 45-50 mph (ish), then firmly brake back to 3,100 rpm and repeat. On a 6 mile downhill run we applied the brakes 5 times. It felt SAFE but was slow ... I'm trading up to a 2016 DRW F350 with a better exhaust brake and a 3.73 rear axle. Learning is happenening.

IMHO, safety subjects like mountain driving can't be discussed enough. Although, in every thread, the advice that is given is sound. Most on here seem to value their lives! The only advice I would like to see reiterated more is "don't worry about what everyone else is doing, slow and safe are the key words, especially when descending". The speed limits posted on roads and curves are usually for cars. Use lower gears and you exhaust brake if you have one. I play a little game when driving across the Grapevine where I am not allowed to touch the chassis brake. Now, I drive it often enough to know how severe each hill is and what it takes to maintain speed but that isn't the point. In all cases, flat or mountain, plan ahead. Evaluate the road, your vehicle, and your skill level. Start down slow. You can always adjust your speed up. Just because a speed demon passes you in his f150 pulling a fully loaded 43' toy hauler doesn't mean it's ok to go that fast. It usually means he is younger and heals faster. Too often I see RVs descending way too fast. Slow, steady and safe, wins the race.