The length limitations are probably a maneuvaribility issue. Those inside sites tend to be smaller, and a lot of RV's have to spend time trying to shoehorn their way in (the campground wasn't built to accomodate them). I don't know that the host or ranger at the kiosk will measure your RV, but built the way they are (one-way loops) an RV trying to get in a spot too small blocks the whole loop until they get in or give up.
If you can get it backed in there, I seriously doubt that a ranger would make you leave unless it sticks out into the road.
Since you already have an RV spot near Yosemite, it may not be worth the trouble, especially if they have any transportation into the park from where you are.
Or, as others said, a cheap tent and sleeping bags (I'd throw in air mattresses too) wouldn't cost a lot, and is a nice way to experience Yosemite, even in the Valley campgrounds; but unless you have a car in addition to the RV, that may not be practical.
Gary
Yosemite Photo Galleries: http://www.pbase.com/roberthouse/yo
