Think of the Sierras as big tilted block. The east side is steeper, so the crest will always be closer to US395 than to the highways on the west side.
US395 lies in a broad dry valley as far north as Bishop, then it crosses several passes, or ridges connecting the Sierras to the White Mtns (on the Nevada border). The area around Mammoth Ski area is the highest.
After 88 around the south end of the Sierras, CA120 is the first road across, passing through Yosemite. US50 is the next RV friendly way across the Sierras (the roads in between are too steep, narrow and/or curvy).
On the west CA99 is the major highway through the populated agricultural Central Valley. There are various highways heading into the Sierra foothills, and some like CA49 that wind their way along the foothills. These are good roads, but some will be challenging in a big RV.