Most campgrounds were first developed in the 1950s and 60s based upon the station wagon and tent camping. Things have changed a bit since then and campgrounds are being redesigned and rebuilt as funds allow. The key phrase is "as funds allow". It's expensive. One popular FS lakefront campground near us was rebuilt to contemporary RV standards at a cost of $1.5 million.
The land agencies are competing with every other agency and program for a limited pool of funds. The campgrounds are being rebuilt to accommodate RVs. It is going to take time to get all of them done.