When is an RV purchase final?

If you havent signed a contract, and still have to do a walkthrough as you stated earlier, you should not be bound. Yes they will likely try to keep your deposit, but for them to try to do any more is unlikely, also your "offer" should have contigincies on it for being approved at the walkthrough. IF the unit doesnt meet your requirements for the walkthrough the dealer should end up in the position of either repairing every single item or having to quit the sale. Its all in your specific deal. I've never dealt with Cali law, but have bought vehicles in a lot of different states, and generally there is a final document called a contract to purchase or something similar, and if you have not signed that, and the dealer has not cleared every item from the problems found prior to delivery, it should be difficult for them to enforce any "offer sheet".
Again its up to your state and their laws, as well as what documents have already been signed.