I-5 doesn't go through SF. It does go through Sacramento, but it's heavily used by trucks, so an RV would be fine, but may have some traffic.
101 goes through SF, and I'd avoid that with a big RV since part of it goes on city streets and some of the lanes are narrow - I've done it in my motorhome, but I wouldn't want to tow anything. If you're coming south, you can cut over on I-580 north of SF to catch I-580 or I-800 south.
If you come in on I-80 into SF, then you can get to 101 south or 280 south while staying on the freeway.