Anything longer than 28' and you will have a lot more difficulty not only finding a space that is long enough but also in having enough room to back in with the trailer. Often the room to back into a space is more of a problem than the overall depth as the "tent area" is not included in the length and some trailers can overhang the end of the parking area.
There is a fantastic book, "California Camping" by Tom Steinstra that anyone traveling around CA should buy. It provides information by geographic area and has excellent maps and is kept current. Much easier to use than the Woodall's directory and much better information include an inventory of spaces by length at different campgrounds.
I extrapolated the information in Tom's book to come up with an idea of how RV length would restrict where we could stay and came up with the following for the area around Yosemite.
16' RV - 211 campgrounds 100% of RV campground
21' RV - 180 campgrounds 85%
25' RV - 150 campgrounds 71%
30' RV - 138 campgrounds 65%
35' RV - 88 campgrounds 42%
40' RV - 64 campgrounds 30%
45' RV - 32 campgrounds 15%