Can't address ALL NFS, but here in Colorado, and specifically the Roosevelt and Arapaho NF that I camp in, nearly all of them have a campground host, and sometimes 2 hosts. Holds true for locations w/no hookups as well as with electric available, and for reservation CG as well as first come/first served campgrounds. All of the ones that I go to regularly have hosts. There's one very remote CG that didn't have a host at the time I went there, but that was many years ago and it may have changed. Most of the CG in this area of Colorado have become very popular and they fill up every weekend during the season.
Regarding Rangers, again with the popular CGs I have seen Rangers, and in addition, the local Sheriff's Department also regularly patrols the campgrounds. This is true even for the CG that are a great distance from any town/city.
As mentioned by a previous post, many of the CG have hosts that work for a vendor that manages multiple CGs. In this area of Colorado, it's American Land and Leisure that provides the hosts. All of the hosts I've talked to love their jobs and return year after year. And most of them have a means to communicate with Rangers and/or law enforcement if needed.