I've never heard of it being necessary to be in the U.S, for 24 hours before renting an RV. However, in LA at least, I'm not sure there's anywhere to rent an RV right at the airport.

Also be aware that RV's can be difficult in the big cities like LA and San Francisco. In LA, for instance, most RV parks are on the outskirts of the city, and using an RV as your mode of transport to get around the city tends to be highly impractical - they can be difficult to drive in city traffic and parking for them can be difficult to find.

So I think your best bet would be to fly to LA, spend a couple of days in LA / Orange Country (search for all-suite hotels like Embassy Suites, Orchid Suites in Hollywood to see if you can find hotels that can handle 6 people in one room) with a rental car if possible. Head to San Diego via car or train. After you leave San Diego, rent an RV and head to the less urban areas. Eventually make your way to San Francisco (via Highway 1 through Big Sur, Monterey, Carmel if possible) and get rid of the RV when you arrive there. You probably won't need a rental car in SF, but again look for suite hotels that can accomodate 6 in one room.

For Surfing, check out Huntington Beach, San Clemente and Pacific Beach/Mission Beach in Southern California.

Huntington Beach, Newport, San Clemente and Dana Point (Doheny Beach) are all places in Southern CA with great surfing. The beaches in Southern CA are very nice, also. Huntington has a pier you can walk out on and is known as "surf city."

Enjoy your travels! :)

Orange county I agree would be a surfer's paradise as far as Ca goes and the water will be a tad bit less cold.

But the monster waves are in one spot in CA which also can be visited by your DH. It is just north of Half Moon Bay, The Mavericks It is 30 or so minutes south of San Francisco and if by chance you are staying at the RV park in Pacifica, it is even closer.

Here is one link to what is going on there this season

http://mavericksinvitational.com/

As far Yosemite, you do not have to rent an RV. There are vacation rentals and hotels that can accommodate six. We stayed at the Cedar Lodge and they have a suite with the kitchen that has a Murphy bed in the living room. And two beds in the other room. That worked out very well.

Thank you everyone for your advice!! Sounds like there are lots of solutions to my worries!

Also wondering if the weather will be warm enough for days on the beaches on the southern end of California? Well warm enough for a welsh family ha!

Not 24 hours but you have to stay overnight first after an intercontinental flight from UK before you can pick up the RV and the p/u will take time not to mention buying groceries.