Not a lot of campgrounds have limits that short for all campsites. Some of the state parks I go to have some campsites that won't take anything longer than a 20 foot van, but those are supposed to be parking for tent campers. Same parks have other sites as deep as 50-60 feet.
Many older public park campgrounds are full of sites built in the 1930s to 1950's for tent campers and small trailer units, and these areas can get quite crowded when all sites fill with large modern RVs with awnings and slideout rooms. I am familiar with one on the south side of Grand Traverse Bay, and I suspect California also has some crowded campgrounds in older parks.
There are options other than campgrounds, particularly disperse camping in forests, where shorter is better and a tent works better than a vehicle of any size.
Pick a RV size based on how you want to use it. That is a compromise between how big a house you think you need and where you want to take it. The smaller it is, the more places you can take it. The bigger it is, the longer you might be willing to live in it.