Have yet to hit a mountain that slowed me down going up, but going down I was a bit more on the cautious side. Driving down a winding mountain road that I was unfamiliar with, I was going to make sure I didn't have a problem. Apparently a little too cautious as I got passed by an old dump truck. That was before I converted to disk brakes in the rear. If you have fear, then slow till the fear goes away.
The old gassers that used to have trouble were the 70s Dodge motorhomes. For what ever reason, they just would not go in thin air. It was not uncommon to see them slowing down to single digits, trying to make it up a mountain. Remember seeing that in Kentucky/Tennessee.