Wow that's a tough one. Mountain roads with climbs and descents no matter how you get there.
For the least amount of time on a mountain road, my first thought would be I-5 south to Hwy 50 east to South Lake. I've done it with a 23ft TT but nothing larger. HWY 50 is a winding road in most places and two lane with some 4-lane sections or passing lanes.
The other option would be I-5 to I-80 east to Donner Pass and then south on Hwy 89. Never done this with an RV only by car. My choice would be Hwy 50 route and take your time.