I've gone that route more times than I can count in both directions with Chevy C3500 pulling a 38 ft 5th wheel (15,000lbs). Going downhill from Gorman there are truck lane and speed restrictions. If you have an exhaust brake it helps tremendously. If no exhaust brake then you need to use what is known as "Aggressive Braking Technique" Use the same gear or lower that you used to climb the hill (I had to use first gear with my truck). My top speed in first was 30MPH so I would only allow the speed to get to 25MPH and then I would use the truck and trailer brakes hard for a few seconds to drop the speed back to 20MPH. Make sure no one is following too close. Let the speed creep up again to 25MPH and then brake again for a few seconds to drop to 20MPH. Keep doing this all the way down the hill to save your brakes.
It's kind of a boring drive along I-5 with not much to see along the way. I actually prefer to go Hwy 101 when headed to Paso Robles area. More to see along the way. It's a good route with only a few hills.
