Personally, I'd come across the Northern route. September won't be too bad. Start with the Giant Redwoods and head south on US101. Lots to see along that route including the Golden Gate. Continue south through Pismo Beach and Morro Bay. Once you get to Watsonville, (Artichoke Capital of the World) head east across Hecker Pass to Gilroy, (Garlic Capital of the World) Don't miss the pier at Santa Cruz. Continue southeast to Fresno and take 180 to Kings Canyon. There are no roads from Kings to US395. Return to US 99 and head north to Yosemite. From there you can pick up US395 and proceed north. Be sure to visit Lake Tahoe. A pleasant side trip is to follow the road completely around the lake. There's an RV park on the Nevada side, Zephyr Cove that's really nice. Use the toad to circle the lake. Take pictures and a picnic lunch. From Reno, you can take US395 until it dead ends at Hwy 20. Turn east and you'll end up in Boise Idaho. If the weather looks bad, take I-80 out of Reno to Salt Lake City.
Obviously there's a bunch more stuff to see in that route. If this is your first trip to California, I'd skip the Kings Canyon leg and use that time to see a lot of the other things along the way......Dennis