Having lived, driven, cycled and walked in California (but never RV'ed), Johnny Faro's post confirms quite a few of my suspicions and observations. A 25 foot/7.5 m vehicle (where do you find an RV that small in the US?) is no problem on main roads, but in cities and on the sort of roads that lead to the more interesting places, you will in fact be driving through places rather than seeing them, and it won't be the enjoyable type of driving. Fuel in California is distinctly more expensive than the rest of the US, although not up to UK levels, and big RVs guzzle a fair bit of the stuff.
Regarding Yosemite and other national parks: An RV would limit your camping options to campsites that you can reserve 6 months in advance (and reserving those campsites is no easy task). The roads into and through some of the more interesting campgrounds (like Tuolumne Meadows in the Yosemite high country) were not built with age vehicles in mind, to put it mildly. Many national parks set aside a certain number of non-reservable campsites for walk-up customers, and many of these are tent only.
Rent a car, buy a pile of camping gear and do your trip that way. At the end of the trip, take the camping gear you want back home with you, and give the rest to Goodwill or a similar charity store. You'll probably save enough money for the occasional motel when it's pissing rain (yes, it's been known to happen in California), and/or the occasional restaurant meal.
If you want ideas on where to go, I've got them.