You can actually get to Sequoia NP from 395, but you have to be in good shape. In Lone Pine take Whitney Portal Road to the west. A few miles outside of town you'll see Horseshoe Meadow Road. Turn left there and follow that road for about 20 miles. Don't take the RV on that road as it's steep and windy and climbs up to about 10,000 feet. At the end of the road you'll find Last Chance Meadow. Park there and then hike about 20 miles west and you'll get to Sequoia National Park. Otherwise, you have to drive around and come in from the West side.
There is a section of the Sierra's between Yosemite (Highway 120) to the North, and Highway 178 to the South, that you can not cross by vehicle. Only airplane, horse, or on foot. Highway 120 closes for the winter along with several other passes so during the winter that uncrossable gap starts at Highway 88/89 in the North and goes down to Highway 178.