As you are particular about the campgrounds where you will stay, I suggest going with the 24ft and buy a popup gazebo that you can put up for extra living space.
Here in Northern California, the foothills and desert areas you can usually fit a 26ft in a 24ft foot space as there are rarely trees, etc preventing you from backing in further. During the winter, these are often nice places to stay as there is no snow and few campers.
In the Sierras, you will have a much harder time trying to fit a 26ft in a 24ft space. the campgrounds that fit your description the best don't fit larger than 20ft and are first come first serve. The next closest (my usual category) will have boulders and/or trees directly behind the parking pad so you cannot back further into the site to fit. Some of the 24ft sites will barely fit a 24ft rig - the front end will be within a foot of the road. And most of my favorite campgrounds in the Sierras don't have any longer sites, the rest at most have 3 or 4 sites for over 24ft and those go very fast.
