I live in the Monterey area and September is the warmest month of the year and a great time to visit. The wine country around Paso Robles and San Luis Obispo is also beautiful with the leaves turning golden and you don't have the crowds or the costs of Napa.
Many places to camp around the state and their is an outstanding book by Tom Stienstra "Moon California Camping: The Complete Guide to More Than 1,400 Tent and RV Campgrounds". He has been camping and traveling and writing about California for 30 years and his books are very informative and well written and the best by far of their kind. I like that he breaks out areas with maps and sub maps unlike other books that often are arranged alphabetically by town and so for an area like Yosemite with other books you need to search on Fresno or Modesto and hope that it covers the area you want to visit. The $16 cost of the book is the best investment you can make for your trip.
The National Forest Service maps for the state can be ordered online and they are mailed to you at very little cost. They show NFS campgrounds that are off the beaten path and there are dozens around Yosemite NP.
Yosemite, Kings Canyon & Sequoia Park and Big Trees will be open. It is a wonderful time of year to visit those areas.
We will be in the Yosemite area at the end of September are are staying at the Yosemite West/Mariposa KOA. The rate is fair and it is close to the park. We are also visiting the Yosemite Mountain Sugar Pine Railroad http://www.ymsprr.com/on that trip.
2009 Dodge Ram 1500 Hemi
2010 Keystone Cougar 244RLS
Food source for one beagle, one terrier mix and two ornery cats
Sequoia RV Ranch is a great place to stay just outside of the National Park. It's along a river and near a quaint little town called Three Rivers.
X2, and it's the nicest campground in the area IMO. Best of all, the owners, Mitch and Janet are great folks and personal friends of many years. They used to be our next door neighbors in Upland, CA. Our kids grew up together. Great folks to deal with and they've really done a spectacular job with the CG.
Sequoia RV Ranch used to be "Trailer Isle" when Mitch and Janet bought the property. "Trailer Isle" was a dump and had a terrible reputation. The owner was never there and they had many squatters living in junk trailers for free. Mitch evicted all the squatters and for about the first 2 months, did nothing but haul garbage and junk out of the place, including junk cars and trailers. He poured a ton of $$ into the place and you would not even recognize it today. Definitely one of the nicest campgrounds I've ever stayed in, and as I mentioned, for sure the nicest in the area. Their river spots with full hookups can't be beat.
The Mammoth lakes, June Lake Loop area in Sept is easy to find a spot as school has started. A day trip to Yosemite valley is a nice day trip from there but not for a MH. There is a lot of info on the net for that area. The lower elevations can be a cooker in Sept so it's beach or hi elevations for me. I will have to Ck out Sequoia Ranch.